[PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Tao Liu]

A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
where image->segment[i].buf might be NULL and copied unchecked.

The NULL buf comes from security/integrity/ima/ima_kexec.c:
ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
but kbuf.buffer is NULL.

Fix this by simply adding a check before copy.

Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Tao Liu <ltao@redhat.com>
---

v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/

---
 arch/riscv/kernel/machine_kexec.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
index 2306ce3e5f22..afc68f6a4aa1 100644
--- a/arch/riscv/kernel/machine_kexec.c
+++ b/arch/riscv/kernel/machine_kexec.c
@@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
 		if (image->segment[i].memsz <= sizeof(fdt))
 			continue;
 
+		/*
+		 * Some segments (e.g. IMA) reserve space but have no buffer
+		 * loaded yet. Skip them as they cannot contain an FDT.
+		 */
+		if (image->segment[i].buf == NULL)
+			continue;
+
 		if (image->file_mode)
 			memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
 		else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
-- 
2.54.0


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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Nutty.Liu]

On 7/1/2026 10:57 AM, Tao Liu wrote:
> A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> where image->segment[i].buf might be NULL and copied unchecked.
>
> The NULL buf comes from security/integrity/ima/ima_kexec.c:
> ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> but kbuf.buffer is NULL.
>
> Fix this by simply adding a check before copy.
>
> Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> Acked-by: Baoquan He <bhe@redhat.com>
> Acked-by: Pratyush Yadav <pratyush@kernel.org>
> Signed-off-by: Tao Liu <ltao@redhat.com>
It would be better to add the log of the dereference calltrace.
Otherwise,
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>

Thanks,
Nutty
> ---
>
> v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
> link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
> link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/
>
> ---
>   arch/riscv/kernel/machine_kexec.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> index 2306ce3e5f22..afc68f6a4aa1 100644
> --- a/arch/riscv/kernel/machine_kexec.c
> +++ b/arch/riscv/kernel/machine_kexec.c
> @@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
>   		if (image->segment[i].memsz <= sizeof(fdt))
>   			continue;
>   
> +		/*
> +		 * Some segments (e.g. IMA) reserve space but have no buffer
> +		 * loaded yet. Skip them as they cannot contain an FDT.
> +		 */
> +		if (image->segment[i].buf == NULL)
> +			continue;
> +
>   		if (image->file_mode)
>   			memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
>   		else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))

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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Jarkko Sakkinen]

On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> where image->segment[i].buf might be NULL and copied unchecked.
> 
> The NULL buf comes from security/integrity/ima/ima_kexec.c:
> ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> but kbuf.buffer is NULL

This should have a proper call sequence. Now the root cause is
obfuscated.

> 
> Fix this by simply adding a check before copy.
> 
> Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> Acked-by: Baoquan He <bhe@redhat.com>
> Acked-by: Pratyush Yadav <pratyush@kernel.org>
> Signed-off-by: Tao Liu <ltao@redhat.com>
> ---
> 
> v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
> link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
> link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/
> 
> ---
>  arch/riscv/kernel/machine_kexec.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> index 2306ce3e5f22..afc68f6a4aa1 100644
> --- a/arch/riscv/kernel/machine_kexec.c
> +++ b/arch/riscv/kernel/machine_kexec.c
> @@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
>  		if (image->segment[i].memsz <= sizeof(fdt))
>  			continue;
>  
> +		/*
> +		 * Some segments (e.g. IMA) reserve space but have no buffer
> +		 * loaded yet. Skip them as they cannot contain an FDT.
> +		 */

This is destined to rot over time. It also adds up also potentially to
the backporting effort while backporting to stable kernes. And most
importantly. Please, don't document every other null check.

> +		if (image->segment[i].buf == NULL)

if (!image->segments[i].buf)

> +			continue;
> +
>  		if (image->file_mode)
>  			memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
>  		else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
> -- 
> 2.54.0
> 
> 

BR, Jarkko

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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Tao Liu]

Hi Jarkko,

On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> > where image->segment[i].buf might be NULL and copied unchecked.
> >
> > The NULL buf comes from security/integrity/ima/ima_kexec.c:
> > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> > but kbuf.buffer is NULL
>
> This should have a proper call sequence. Now the root cause is
> obfuscated.

Sure, I will attach the stack trace in v4. Here is the one:

[   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
bufsz=0x34ed800 memsz=0x35d0000
[   62.879983] Unable to handle kernel access to user memory without
uaccess routines at virtual address 0000000000000000
[   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
pgdp=0x00000001062eb000
[   62.881185] [0000000000000000] pgd=00000000413b4401,
p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
pte=0000000000000000
[   62.881969] Oops [#1]
[   62.882077] Modules linked in:
[   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
PREEMPT(lazy)
[   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
edk2-20260508-2.fc44 05/08/2026
[   62.883365] epc : __memcpy+0xd4/0xf8
[   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
[   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
ff20000004a83d10
[   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
ffffffff80033640
[   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
ff20000004a83d80
[   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
0000000000000000
[   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
0000000000000000
[   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
0000000000000000
[   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
0000000000000010
[   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
0000000000000005
[   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
ffffffff835420e8
[   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
ff1c000002138d00
[   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
0000000000000000
[   62.886860] status: 0000000200000120 badaddr: 0000000000000000
cause: 000000000000000d
[   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
[   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
[   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
[   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
[   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
[   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
(4198) 0591
[   62.889223] ---[ end trace 0000000000000000 ]---
Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
--initrd=/boot/initramfs-7.1.1.img --reuse-cmdline

(gdb) p image->segment[0]
$3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}

The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
I'm not sure why it added a NULL segment, but it is no harm to add a
NULL checker here in case any other scenarios similar as IMA.

>
> >
> > Fix this by simply adding a check before copy.
> >
> > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> > Acked-by: Baoquan He <bhe@redhat.com>
> > Acked-by: Pratyush Yadav <pratyush@kernel.org>
> > Signed-off-by: Tao Liu <ltao@redhat.com>
> > ---
> >
> > v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
> > link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
> > link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/
> >
> > ---
> >  arch/riscv/kernel/machine_kexec.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> > index 2306ce3e5f22..afc68f6a4aa1 100644
> > --- a/arch/riscv/kernel/machine_kexec.c
> > +++ b/arch/riscv/kernel/machine_kexec.c
> > @@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
> >               if (image->segment[i].memsz <= sizeof(fdt))
> >                       continue;
> >
> > +             /*
> > +              * Some segments (e.g. IMA) reserve space but have no buffer
> > +              * loaded yet. Skip them as they cannot contain an FDT.
> > +              */
>
> This is destined to rot over time. It also adds up also potentially to
> the backporting effort while backporting to stable kernes. And most
> importantly. Please, don't document every other null check.

OK, will get rid of it.

Thanks,
Tao Liu

>
> > +             if (image->segment[i].buf == NULL)
>
> if (!image->segments[i].buf)
>
> > +                     continue;
> > +
> >               if (image->file_mode)
> >                       memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
> >               else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
> > --
> > 2.54.0
> >
> >
>
> BR, Jarkko
>


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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Jarkko Sakkinen]

On Wed, Jul 01, 2026 at 04:58:09PM +1200, Tao Liu wrote:
> Hi Jarkko,
> 
> On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
> >
> > On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> > > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> > > where image->segment[i].buf might be NULL and copied unchecked.
> > >
> > > The NULL buf comes from security/integrity/ima/ima_kexec.c:
> > > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> > > but kbuf.buffer is NULL
> >
> > This should have a proper call sequence. Now the root cause is
> > obfuscated.
> 
> Sure, I will attach the stack trace in v4. Here is the one:
> 
> [   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
> bufsz=0x34ed800 memsz=0x35d0000
> [   62.879983] Unable to handle kernel access to user memory without
> uaccess routines at virtual address 0000000000000000
> [   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
> pgdp=0x00000001062eb000
> [   62.881185] [0000000000000000] pgd=00000000413b4401,
> p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
> pte=0000000000000000
> [   62.881969] Oops [#1]
> [   62.882077] Modules linked in:
> [   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
> PREEMPT(lazy)
> [   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
> edk2-20260508-2.fc44 05/08/2026
> [   62.883365] epc : __memcpy+0xd4/0xf8
> [   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
> [   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
> ff20000004a83d10
> [   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
> ffffffff80033640
> [   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
> ff20000004a83d80
> [   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
> 0000000000000000
> [   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
> 0000000000000000
> [   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
> 0000000000000000
> [   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
> 0000000000000010
> [   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
> 0000000000000005
> [   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
> ffffffff835420e8
> [   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
> ff1c000002138d00
> [   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
> 0000000000000000
> [   62.886860] status: 0000000200000120 badaddr: 0000000000000000
> cause: 000000000000000d
> [   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
> [   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
> [   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
> [   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
> [   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
> [   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
> (4198) 0591
> [   62.889223] ---[ end trace 0000000000000000 ]---
> Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
> --initrd=/boot/initramfs-7.1.1.img --reuse-cmdline
> 
> (gdb) p image->segment[0]
> $3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}
> 
> The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
> I'm not sure why it added a NULL segment, but it is no harm to add a
> NULL checker here in case any other scenarios similar as IMA.

The mission oriented purpose of kernel's git log is to be a quick
checklist for bisecting bugs for instance. You have a dump of details
there for a transcript. Now you should do rationalize that because how
can you otherwise trust your own code?

Here's one recent example from me:

https://lore.kernel.org/linux-integrity/20260509185108.2681198-1-jarkko@kernel.org/

This is IMHO way more important part for a bug fix like this than the
code change itself.

It's the "engineering part" of the  equation.

> 
> >
> > >
> > > Fix this by simply adding a check before copy.
> > >
> > > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> > > Acked-by: Baoquan He <bhe@redhat.com>
> > > Acked-by: Pratyush Yadav <pratyush@kernel.org>
> > > Signed-off-by: Tao Liu <ltao@redhat.com>
> > > ---
> > >
> > > v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
> > > link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
> > > link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/
> > >
> > > ---
> > >  arch/riscv/kernel/machine_kexec.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> > > index 2306ce3e5f22..afc68f6a4aa1 100644
> > > --- a/arch/riscv/kernel/machine_kexec.c
> > > +++ b/arch/riscv/kernel/machine_kexec.c
> > > @@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
> > >               if (image->segment[i].memsz <= sizeof(fdt))
> > >                       continue;
> > >
> > > +             /*
> > > +              * Some segments (e.g. IMA) reserve space but have no buffer
> > > +              * loaded yet. Skip them as they cannot contain an FDT.
> > > +              */
> >
> > This is destined to rot over time. It also adds up also potentially to
> > the backporting effort while backporting to stable kernes. And most
> > importantly. Please, don't document every other null check.
> 
> OK, will get rid of it.

general rules of thumb i personally follow usually:

1. features/improvements: can be a bit more eager with comments
2. bugs: you really have to have rationale for having a commnet. E.g.,
   mandatory SAFETY comment in linux-rust would obviously qualify.
3. both: if you add a comment are you sure it won't rot over time?

> 
> Thanks,
> Tao Liu
> 
> >
> > > +             if (image->segment[i].buf == NULL)
> >
> > if (!image->segments[i].buf)
> >
> > > +                     continue;
> > > +
> > >               if (image->file_mode)
> > >                       memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
> > >               else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
> > > --
> > > 2.54.0
> > >
> > >
> >
> > BR, Jarkko
> >
> 

BR, Jarkko

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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Tao Liu]

Hi Jarkko,

On Wed, Jul 1, 2026 at 10:34 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Wed, Jul 01, 2026 at 04:58:09PM +1200, Tao Liu wrote:
> > Hi Jarkko,
> >
> > On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
> > >
> > > On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> > > > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> > > > where image->segment[i].buf might be NULL and copied unchecked.
> > > >
> > > > The NULL buf comes from security/integrity/ima/ima_kexec.c:
> > > > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> > > > but kbuf.buffer is NULL
> > >
> > > This should have a proper call sequence. Now the root cause is
> > > obfuscated.
> >
> > Sure, I will attach the stack trace in v4. Here is the one:
> >
> > [   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
> > bufsz=0x34ed800 memsz=0x35d0000
> > [   62.879983] Unable to handle kernel access to user memory without
> > uaccess routines at virtual address 0000000000000000
> > [   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
> > pgdp=0x00000001062eb000
> > [   62.881185] [0000000000000000] pgd=00000000413b4401,
> > p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
> > pte=0000000000000000
> > [   62.881969] Oops [#1]
> > [   62.882077] Modules linked in:
> > [   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
> > PREEMPT(lazy)
> > [   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
> > edk2-20260508-2.fc44 05/08/2026
> > [   62.883365] epc : __memcpy+0xd4/0xf8
> > [   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
> > [   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
> > ff20000004a83d10
> > [   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
> > ffffffff80033640
> > [   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
> > ff20000004a83d80
> > [   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
> > 0000000000000000
> > [   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
> > 0000000000000000
> > [   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
> > 0000000000000000
> > [   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
> > 0000000000000010
> > [   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
> > 0000000000000005
> > [   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
> > ffffffff835420e8
> > [   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
> > ff1c000002138d00
> > [   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
> > 0000000000000000
> > [   62.886860] status: 0000000200000120 badaddr: 0000000000000000
> > cause: 000000000000000d
> > [   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
> > [   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
> > [   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
> > [   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
> > [   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
> > [   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
> > (4198) 0591
> > [   62.889223] ---[ end trace 0000000000000000 ]---
> > Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
> > --initrd=/boot/initramfs-7.1.1.img --reuse-cmdline
> >
> > (gdb) p image->segment[0]
> > $3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}
> >
> > The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
> > I'm not sure why it added a NULL segment, but it is no harm to add a
> > NULL checker here in case any other scenarios similar as IMA.
>
> The mission oriented purpose of kernel's git log is to be a quick
> checklist for bisecting bugs for instance. You have a dump of details
> there for a transcript. Now you should do rationalize that because how
> can you otherwise trust your own code?
>
> Here's one recent example from me:
>
> https://lore.kernel.org/linux-integrity/20260509185108.2681198-1-jarkko@kernel.org/
>
> This is IMHO way more important part for a bug fix like this than the
> code change itself.
>
> It's the "engineering part" of the  equation.
>
> >
> > >
> > > >
> > > > Fix this by simply adding a check before copy.
> > > >
> > > > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> > > > Acked-by: Baoquan He <bhe@redhat.com>
> > > > Acked-by: Pratyush Yadav <pratyush@kernel.org>
> > > > Signed-off-by: Tao Liu <ltao@redhat.com>
> > > > ---
> > > >
> > > > v3 -> v2: Add fixes tag; Replace "reference" to "dereference".
> > > > link to v2: https://lore.kernel.org/linux-riscv/20260627222602.23594-2-ltao@redhat.com/
> > > > link to v1: https://lore.kernel.org/linux-riscv/20260529032739.13264-2-ltao@redhat.com/
> > > >
> > > > ---
> > > >  arch/riscv/kernel/machine_kexec.c | 7 +++++++
> > > >  1 file changed, 7 insertions(+)
> > > >
> > > > diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> > > > index 2306ce3e5f22..afc68f6a4aa1 100644
> > > > --- a/arch/riscv/kernel/machine_kexec.c
> > > > +++ b/arch/riscv/kernel/machine_kexec.c
> > > > @@ -41,6 +41,13 @@ machine_kexec_prepare(struct kimage *image)
> > > >               if (image->segment[i].memsz <= sizeof(fdt))
> > > >                       continue;
> > > >
> > > > +             /*
> > > > +              * Some segments (e.g. IMA) reserve space but have no buffer
> > > > +              * loaded yet. Skip them as they cannot contain an FDT.
> > > > +              */
> > >
> > > This is destined to rot over time. It also adds up also potentially to
> > > the backporting effort while backporting to stable kernes. And most
> > > importantly. Please, don't document every other null check.
> >
> > OK, will get rid of it.
>
> general rules of thumb i personally follow usually:
>
> 1. features/improvements: can be a bit more eager with comments
> 2. bugs: you really have to have rationale for having a commnet. E.g.,
>    mandatory SAFETY comment in linux-rust would obviously qualify.
> 3. both: if you add a comment are you sure it won't rot over time?
>

Thank you very much for providing detailed guidance on code commenting
and commit log drafting. I really appreciate your help with those!

Thanks,
Tao Liu

> >
> > Thanks,
> > Tao Liu
> >
> > >
> > > > +             if (image->segment[i].buf == NULL)
> > >
> > > if (!image->segments[i].buf)
> > >
> > > > +                     continue;
> > > > +
> > > >               if (image->file_mode)
> > > >                       memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
> > > >               else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
> > > > --
> > > > 2.54.0
> > > >
> > > >
> > >
> > > BR, Jarkko
> > >
> >
>
> BR, Jarkko
>


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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Pratyush Yadav]

On Wed, Jul 01 2026, Tao Liu wrote:

> Hi Jarkko,
>
> On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>>
>> On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
>> > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
>> > where image->segment[i].buf might be NULL and copied unchecked.
>> >
>> > The NULL buf comes from security/integrity/ima/ima_kexec.c:
>> > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
>> > but kbuf.buffer is NULL
>>
>> This should have a proper call sequence. Now the root cause is
>> obfuscated.
>
> Sure, I will attach the stack trace in v4. Here is the one:
>
> [   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
> bufsz=0x34ed800 memsz=0x35d0000
> [   62.879983] Unable to handle kernel access to user memory without
> uaccess routines at virtual address 0000000000000000
> [   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
> pgdp=0x00000001062eb000
> [   62.881185] [0000000000000000] pgd=00000000413b4401,
> p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
> pte=0000000000000000
> [   62.881969] Oops [#1]
> [   62.882077] Modules linked in:
> [   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
> PREEMPT(lazy)
> [   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
> edk2-20260508-2.fc44 05/08/2026
> [   62.883365] epc : __memcpy+0xd4/0xf8
> [   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
> [   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
> ff20000004a83d10
> [   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
> ffffffff80033640
> [   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
> ff20000004a83d80
> [   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
> 0000000000000000
> [   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
> 0000000000000000
> [   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
> 0000000000000000
> [   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
> 0000000000000010
> [   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
> 0000000000000005
> [   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
> ffffffff835420e8
> [   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
> ff1c000002138d00
> [   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
> 0000000000000000
> [   62.886860] status: 0000000200000120 badaddr: 0000000000000000
> cause: 000000000000000d
> [   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
> [   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
> [   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
> [   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
> [   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
> [   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
> (4198) 0591
> [   62.889223] ---[ end trace 0000000000000000 ]---
> Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
> --initrd=/boot/initramfs-7.1.1.img --reuse-cmdline
>
> (gdb) p image->segment[0]
> $3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}
>
> The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
> I'm not sure why it added a NULL segment, but it is no harm to add a
> NULL checker here in case any other scenarios similar as IMA.

From my reading of the IMA code, I think they do so because they don't
want to add a buffer to the kimage just yet. This is because there can
be IMA measurements between kexec load and kexec reboot. So they use
kexec_add_buffer() to reserve the physical address space, then they vmap
the destination pages in ima_kexec_post_load(), and then update the
destination pages directly via a reboot notifier. So they do not have a
buffer copied to the destination pages at the time of
kexec_add_buffer().

Now all that is fairly convoluted and not very obvious, but I am not
sure if there is a better way of doing this off the top of my head.

>
>>
>> >
>> > Fix this by simply adding a check before copy.
>> >
>> > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
>> > Acked-by: Baoquan He <bhe@redhat.com>
>> > Acked-by: Pratyush Yadav <pratyush@kernel.org>
>> > Signed-off-by: Tao Liu <ltao@redhat.com>
[...]

-- 
Regards,
Pratyush Yadav

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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Tao Liu]

Hi Pratyush,

On Thu, Jul 2, 2026 at 12:06 AM Pratyush Yadav <pratyush@kernel.org> wrote:
>
> On Wed, Jul 01 2026, Tao Liu wrote:
>
> > Hi Jarkko,
> >
> > On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
> >>
> >> On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> >> > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> >> > where image->segment[i].buf might be NULL and copied unchecked.
> >> >
> >> > The NULL buf comes from security/integrity/ima/ima_kexec.c:
> >> > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> >> > but kbuf.buffer is NULL
> >>
> >> This should have a proper call sequence. Now the root cause is
> >> obfuscated.
> >
> > Sure, I will attach the stack trace in v4. Here is the one:
> >
> > [   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
> > bufsz=0x34ed800 memsz=0x35d0000
> > [   62.879983] Unable to handle kernel access to user memory without
> > uaccess routines at virtual address 0000000000000000
> > [   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
> > pgdp=0x00000001062eb000
> > [   62.881185] [0000000000000000] pgd=00000000413b4401,
> > p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
> > pte=0000000000000000
> > [   62.881969] Oops [#1]
> > [   62.882077] Modules linked in:
> > [   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
> > PREEMPT(lazy)
> > [   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
> > edk2-20260508-2.fc44 05/08/2026
> > [   62.883365] epc : __memcpy+0xd4/0xf8
> > [   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
> > [   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
> > ff20000004a83d10
> > [   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
> > ffffffff80033640
> > [   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
> > ff20000004a83d80
> > [   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
> > 0000000000000000
> > [   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
> > 0000000000000000
> > [   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
> > 0000000000000000
> > [   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
> > 0000000000000010
> > [   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
> > 0000000000000005
> > [   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
> > ffffffff835420e8
> > [   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
> > ff1c000002138d00
> > [   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
> > 0000000000000000
> > [   62.886860] status: 0000000200000120 badaddr: 0000000000000000
> > cause: 000000000000000d
> > [   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
> > [   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
> > [   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
> > [   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
> > [   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
> > [   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
> > (4198) 0591
> > [   62.889223] ---[ end trace 0000000000000000 ]---
> > Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
> > --initrd=/boot/initramfs-7.1.1.img --reuse-cmdline
> >
> > (gdb) p image->segment[0]
> > $3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}
> >
> > The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
> > I'm not sure why it added a NULL segment, but it is no harm to add a
> > NULL checker here in case any other scenarios similar as IMA.
>
> From my reading of the IMA code, I think they do so because they don't
> want to add a buffer to the kimage just yet. This is because there can
> be IMA measurements between kexec load and kexec reboot. So they use
> kexec_add_buffer() to reserve the physical address space, then they vmap
> the destination pages in ima_kexec_post_load(), and then update the
> destination pages directly via a reboot notifier. So they do not have a
> buffer copied to the destination pages at the time of
> kexec_add_buffer().
>
> Now all that is fairly convoluted and not very obvious, but I am not
> sure if there is a better way of doing this off the top of my head.

Thank you very much for resolving my confusion! Now I understand it
much better...

Thanks,
Tao Liu

>
> >
> >>
> >> >
> >> > Fix this by simply adding a check before copy.
> >> >
> >> > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> >> > Acked-by: Baoquan He <bhe@redhat.com>
> >> > Acked-by: Pratyush Yadav <pratyush@kernel.org>
> >> > Signed-off-by: Tao Liu <ltao@redhat.com>
> [...]
>
> --
> Regards,
> Pratyush Yadav
>


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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Markus Elfring]

…
> Fix this by simply adding a check before copy.

                                           a data copy attempt?



> Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")

See also once more:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/stable-kernel-rules.rst?h=v7.2-rc1#n34


Would a summary phrase like “Prevent null pointer dereference in machine_kexec_prepare()”
be nicer?

Regards,
Markus

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Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

[Tao Liu]

Hi Markus,

On Wed, Jul 1, 2026 at 6:01 PM Markus Elfring <Markus.Elfring@web.de> wrote:
>
> …
> > Fix this by simply adding a check before copy.
>
>                                            a data copy attempt?
>
>
>
> > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
>
> See also once more:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/stable-kernel-rules.rst?h=v7.2-rc1#n34
>
>
> Would a summary phrase like “Prevent null pointer dereference in machine_kexec_prepare()”
> be nicer?

Agreed, I will improve it in v4. Thanks again for your guidance!

Thanks,
Tao Liu

>
> Regards,
> Markus
>


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