Kernel panic in fs compat layer

Kernel panic in fs compat layer

[Helge Deller]

Looping in the linux-fsdevel mailing list in this mail....

Dave faced a kernel crash on parisc with 32-bit userspace on 64-bit kernel.
It seems the patch below fixes this bug.

Helge

* John David Anglin <dave.anglin@bell.net>:
> On 2017-08-19, at 8:56 AM, John David Anglin wrote:
> 
> > mx3210 login: systemd-logind[4461]: New seat seat0.
> > systemd-logind[4461]: Failed to start user service, ignoring: Unknown unit: user@0.service
> > systemd-logind[4461]: Failed to start user service, ignoring: Unknown unit: user@110.service
> > Backtrace:
> > [<00000000402f316c>] compat_get_fd_set+0x5c/0x78
> > [<00000000402f3cac>] compat_core_sys_select+0x1cc/0x300
> > [<00000000402f52dc>] compat_SyS_select+0x144/0x1a0
> > [<0000000040155fe4>] syscall_exit+0x0/0x14
> > 
> > 
> > Kernel Fault: Code=26 (Data memory access rights trap) regs=00000002234b84e0 (Addr=0000000000000000)
> > CPU: 1 PID: 21167 Comm: polyimport Not tainted 4.13.0-rc5+ #1
> > task: 0000000223d74b50 task.stack: 00000002234b8000
> > 
> > YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI
> > PSW: 00001000000001101111111100001111 Not tainted
> > r00-03 000000ff0806ff0f 00000002234b8420 00000000402f316c 00000002234b84a0
> > r04-07 000000004071c960 00000002234b82b8 00000002234b8218 0000000000000100
> > r08-11 0000000000000004 0000000000000100 00000000f3780588 0000000000000000
> > r12-15 0000000000000000 00000002234b8148 0000000000000000 00000002234b8218
> > r16-19 0000000000000000 000000000002f03c 000000000002f030 0000000000000000
> > r20-23 0000000000000100 0000000000000000 00000002234b8148 0000000000000000
> > r24-27 0000000000000100 0000000000000000 0000000000000000 000000004071c960
> > r28-31 0000000000000000 00000002234b8470 00000002234b84e0 0000000000000000
> > sr00-03 0000000003f15000 0000000003f15000 0000000003f15000 0000000003f15000
> > sr04-07 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> > 
> > IASQ: 0000000000000000 0000000000000000 IAOQ: 00000000406a4718 00000000406a471c
> > IIR: 0ff312c0 ISR: 0000000000000000 IOR: 0000000000000000
> > CPU: 1 CR30: 00000002234b8000 CR31: b33e06ff0008e07f
> > ORIG_R28: 0000000000000040
> > IAOQ[0]: memset+0x68/0xd8
> > IAOQ[1]: memset+0x6c/0xd8
> > RP(r2): compat_get_fd_set+0x5c/0x78
> > Backtrace:
> > [<00000000402f316c>] compat_get_fd_set+0x5c/0x78
> > [<00000000402f3cac>] compat_core_sys_select+0x1cc/0x300
> > [<00000000402f52dc>] compat_SyS_select+0x144/0x1a0
> > [<0000000040155fe4>] syscall_exit+0x0/0x14
> > 
> > Kernel panic - not syncing: Kernel Fault
> > ---[ end Kernel panic - not syncing: Kernel Fault
> 
> 
> The attached patch is probably not the real fix but it should prevent the panic.
> 
> 

> diff --git a/fs/select.c b/fs/select.c
> index d6c652a31e99..8790c0a0bd3c 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -1181,6 +1181,8 @@ int compat_get_fd_set(unsigned long nr, compat_ulong_t __user *ufdset,
>  		if (odd && __get_user(*fdset, ufdset))
>  			return -EFAULT;
>  	} else {
> +		if (!fdset)
> +			return -EFAULT;
>  		/* Tricky, must clear full unsigned long in the
>  		 * kernel fdset at the end, this makes sure that
>  		 * actually happens.

[PATCH] fs/select: Fix kernel panic due to memory corruption in compat_get_fd_set()

[Helge Deller]

Dave, can you test this patch?
Helge

------
[PATCH] Fix kernel panic due to memory corruption in compat_get_fd_set()

Dave faced this kernel crash in the select() compat syscall on the
parisc architecture:

 Kernel Fault: Code=26 (Data memory access rights trap) regs=00000002234b84e0 (Addr=0000000000000000)
 CPU: 1 PID: 21167 Comm: polyimport Not tainted 4.13.0-rc5+ #1
 task: 0000000223d74b50 task.stack: 00000002234b8000
 IAOQ[0]: memset+0x68/0xd8
 IAOQ[1]: memset+0x6c/0xd8
 RP(r2): compat_get_fd_set+0x5c/0x78
 Backtrace:
 [<00000000402f316c>] compat_get_fd_set+0x5c/0x78
 [<00000000402f3cac>] compat_core_sys_select+0x1cc/0x300
 [<00000000402f52dc>] compat_SyS_select+0x144/0x1a0
 [<0000000040155fe4>] syscall_exit+0x0/0x14
 Kernel panic - not syncing: Kernel Fault

It seems commit 464d62421cb8 ("select: switch compat_{get,put}_fd_set()
to compat_{get,put}_bitmap()") broke the calculation on how many bytes
need to be zeroed in case of fdset beeing NULL. This leads to memory
overwrites and crashes.

Fixes: 464d62421cb8 ("select: switch compat_{get,put}_fd_set() to compat_{get,put}_bitmap()"
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/fs/select.c b/fs/select.c
index 9d5f15e..c6362e3 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -1164,11 +1164,7 @@ int compat_get_fd_set(unsigned long nr, compat_ulong_t __user *ufdset,
 	if (ufdset) {
 		return compat_get_bitmap(fdset, ufdset, nr);
 	} else {
-		/* Tricky, must clear full unsigned long in the
-		 * kernel fdset at the end, ALIGN makes sure that
-		 * actually happens.
-		 */
-		memset(fdset, 0, ALIGN(nr, BITS_PER_LONG));
+		zero_fd_set(nr, fdset);
 		return 0;
 	}
 }