[PATCH v2] fs/proc: check p->vec_buf for NULL

[PATCH v2] fs/proc: check p->vec_buf for NULL

[Jakub Acs]

When PAGEMAP_SCAN ioctl invoked with vec_len = 0 reaches
pagemap_scan_backout_range(), kernel panics with null-ptr-deref:

[   44.936808] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
[   44.937797] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   44.938391] CPU: 1 UID: 0 PID: 2480 Comm: reproducer Not tainted 6.17.0-rc6 #22 PREEMPT(none)
[   44.939062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   44.939935] RIP: 0010:pagemap_scan_thp_entry.isra.0+0x741/0xa80

<snip registers, unreliable trace>

[   44.946828] Call Trace:
[   44.947030]  <TASK>
[   44.949219]  pagemap_scan_pmd_entry+0xec/0xfa0
[   44.952593]  walk_pmd_range.isra.0+0x302/0x910
[   44.954069]  walk_pud_range.isra.0+0x419/0x790
[   44.954427]  walk_p4d_range+0x41e/0x620
[   44.954743]  walk_pgd_range+0x31e/0x630
[   44.955057]  __walk_page_range+0x160/0x670
[   44.956883]  walk_page_range_mm+0x408/0x980
[   44.958677]  walk_page_range+0x66/0x90
[   44.958984]  do_pagemap_scan+0x28d/0x9c0
[   44.961833]  do_pagemap_cmd+0x59/0x80
[   44.962484]  __x64_sys_ioctl+0x18d/0x210
[   44.962804]  do_syscall_64+0x5b/0x290
[   44.963111]  entry_SYSCALL_64_after_hwframe+0x76/0x7e

vec_len = 0 in pagemap_scan_init_bounce_buffer() means no buffers are
allocated and p->vec_buf remains set to NULL.

This breaks an assumption made later in pagemap_scan_backout_range(),
that page_region is always allocated for p->vec_buf_index.

Fix it by explicitly checking p->vec_buf for NULL before dereferencing.

Other sites that might run into same deref-issue are already (directly
or transitively) protected by checking p->vec_buf.

Note:
From PAGEMAP_SCAN man page, it seems vec_len = 0 is valid when no output
is requested and it's only the side effects caller is interested in,
hence it passes check in pagemap_scan_get_args().

This issue was found by syzkaller.

Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Jakub Acs <acsjakub@amazon.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Penglei Jiang <superman.xpt@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
---
 fs/proc/task_mmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 29cca0e6d0ff..b26ae556b446 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -2417,6 +2417,9 @@ static void pagemap_scan_backout_range(struct pagemap_scan_private *p,
 {
 	struct page_region *cur_buf = &p->vec_buf[p->vec_buf_index];
 
+	if (!p->vec_buf)
+		return;
+
 	if (cur_buf->start != addr)
 		cur_buf->end = addr;
 	else
-- 
2.47.3




Amazon Web Services Development Center Germany GmbH
Tamara-Danz-Str. 13
10243 Berlin
Geschaeftsfuehrung: Christian Schlaeger
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
Sitz: Berlin
Ust-ID: DE 365 538 597

Re: [PATCH v2] fs/proc: check p->vec_buf for NULL

[David Hildenbrand]

On 22.09.25 10:17, Jakub Acs wrote:
> When PAGEMAP_SCAN ioctl invoked with vec_len = 0 reaches
> pagemap_scan_backout_range(), kernel panics with null-ptr-deref:
> 
> [   44.936808] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
> [   44.937797] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> [   44.938391] CPU: 1 UID: 0 PID: 2480 Comm: reproducer Not tainted 6.17.0-rc6 #22 PREEMPT(none)
> [   44.939062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> [   44.939935] RIP: 0010:pagemap_scan_thp_entry.isra.0+0x741/0xa80
> 
> <snip registers, unreliable trace>
> 
> [   44.946828] Call Trace:
> [   44.947030]  <TASK>
> [   44.949219]  pagemap_scan_pmd_entry+0xec/0xfa0
> [   44.952593]  walk_pmd_range.isra.0+0x302/0x910
> [   44.954069]  walk_pud_range.isra.0+0x419/0x790
> [   44.954427]  walk_p4d_range+0x41e/0x620
> [   44.954743]  walk_pgd_range+0x31e/0x630
> [   44.955057]  __walk_page_range+0x160/0x670
> [   44.956883]  walk_page_range_mm+0x408/0x980
> [   44.958677]  walk_page_range+0x66/0x90
> [   44.958984]  do_pagemap_scan+0x28d/0x9c0
> [   44.961833]  do_pagemap_cmd+0x59/0x80
> [   44.962484]  __x64_sys_ioctl+0x18d/0x210
> [   44.962804]  do_syscall_64+0x5b/0x290
> [   44.963111]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> vec_len = 0 in pagemap_scan_init_bounce_buffer() means no buffers are
> allocated and p->vec_buf remains set to NULL.
> 
> This breaks an assumption made later in pagemap_scan_backout_range(),
> that page_region is always allocated for p->vec_buf_index.
> 
> Fix it by explicitly checking p->vec_buf for NULL before dereferencing.
> 
> Other sites that might run into same deref-issue are already (directly
> or transitively) protected by checking p->vec_buf.
> 
> Note:
>  From PAGEMAP_SCAN man page, it seems vec_len = 0 is valid when no output
> is requested and it's only the side effects caller is interested in,
> hence it passes check in pagemap_scan_get_args().
> 
> This issue was found by syzkaller.
> 

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb

Re: [PATCH v2] fs/proc: check p->vec_buf for NULL

[Andrei Vagin]

On Mon, Sep 22, 2025 at 1:17 AM Jakub Acs <acsjakub@amazon.de> wrote:
>
> When PAGEMAP_SCAN ioctl invoked with vec_len = 0 reaches
> pagemap_scan_backout_range(), kernel panics with null-ptr-deref:
>
> [   44.936808] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
> [   44.937797] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> [   44.938391] CPU: 1 UID: 0 PID: 2480 Comm: reproducer Not tainted 6.17.0-rc6 #22 PREEMPT(none)
> [   44.939062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> [   44.939935] RIP: 0010:pagemap_scan_thp_entry.isra.0+0x741/0xa80
>
> <snip registers, unreliable trace>
>
> [   44.946828] Call Trace:
> [   44.947030]  <TASK>
> [   44.949219]  pagemap_scan_pmd_entry+0xec/0xfa0
> [   44.952593]  walk_pmd_range.isra.0+0x302/0x910
> [   44.954069]  walk_pud_range.isra.0+0x419/0x790
> [   44.954427]  walk_p4d_range+0x41e/0x620
> [   44.954743]  walk_pgd_range+0x31e/0x630
> [   44.955057]  __walk_page_range+0x160/0x670
> [   44.956883]  walk_page_range_mm+0x408/0x980
> [   44.958677]  walk_page_range+0x66/0x90
> [   44.958984]  do_pagemap_scan+0x28d/0x9c0
> [   44.961833]  do_pagemap_cmd+0x59/0x80
> [   44.962484]  __x64_sys_ioctl+0x18d/0x210
> [   44.962804]  do_syscall_64+0x5b/0x290
> [   44.963111]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> vec_len = 0 in pagemap_scan_init_bounce_buffer() means no buffers are
> allocated and p->vec_buf remains set to NULL.
>
> This breaks an assumption made later in pagemap_scan_backout_range(),
> that page_region is always allocated for p->vec_buf_index.
>
> Fix it by explicitly checking p->vec_buf for NULL before dereferencing.
>
> Other sites that might run into same deref-issue are already (directly
> or transitively) protected by checking p->vec_buf.
>
> Note:
> From PAGEMAP_SCAN man page, it seems vec_len = 0 is valid when no output
> is requested and it's only the side effects caller is interested in,
> hence it passes check in pagemap_scan_get_args().
>
> This issue was found by syzkaller.
>
> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
> Signed-off-by: Jakub Acs <acsjakub@amazon.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Jinjiang Tu <tujinjiang@huawei.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Penglei Jiang <superman.xpt@gmail.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Andrei Vagin <avagin@gmail.com>

Acked-by:  Andrei Vagin <avagin@gmail.com>

Thanks,
Andrei