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SubjectRe: [PATCH] x86/rwsem: Save and restore all callee-clobbered regs in 32-bit ____down_write()
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On 05/12/2016 10:29 AM, Borislav Petkov wrote:
> Anyway, here's an actual patch with a commit message. Guenter, can you
> give it a run please?
>
> It does fix the issue here with your .config but I'd appreciate a
> confirmation.
>
> Thanks.
>
> ---
> From: Borislav Petkov <bp@suse.de>
>
> ____down_write() calls a function to handle the slow path when the lock
> is contended. But in order to be able to call a C function, one has to
> stash all callee-clobbered registers. The 32-bit path saves only %ecx
> for a reason unknown to me. However, after
>
> 71c01930b42e ("locking/rwsem, x86: Clean up ____down_write()")
>
> the useless dependency on edx was removed and this caused the following
> splat:
>
> BUG: unable to handle kernel NULL pointer dereference at 00000015
> IP: [<c185e094>] down_write+0x24/0x30
> *pde = 00000000
> Oops: 0002 [#1] PREEMPT SMP
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper/0 Tainted: G S W 4.6.0-rc7-next-20160511-yocto-standard #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
> task: f4d00000 ti: f4d08000 task.ti: f4d08000
> EIP: 0060:[<c185e094>] EFLAGS: 00210282 CPU: 0
> EIP is at down_write+0x24/0x30
> EAX: f4d00000 EBX: f4f6d600 ECX: ffff0001 EDX: 00000001
> ESI: 00000168 EDI: c1c2eb68 EBP: f4d09ef4 ESP: f4d09eec
> DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> CR0: 80050033 CR2: 00000015 CR3: 01ccb000 CR4: 000406d0
>
> This happens because gcc decided to stash the pointer to @sem in edx (it
> is not used in the inline asm anymore, thus free):
>
> movl %eax, %edx # sem, sem
>
> lock; xadd %ecx,(%eax) # tmp91, sem
> test ...
>
> call call_rwsem_down_write_failed
>
> mov %eax,0x14(%edx)
>
> *before* the slow path happens and if we hit it on 32-bit, it can
> clobber edx and we're staring at garbage value at deref time.
>
> The simple fix is to save/restore edx too, around the slow path. We
> don't need to stash eax because it is used in the slow path as the @sem
> arg.
>
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: x86@kernel.org
> Signed-off-by: Borislav Petkov <bp@suse.de>

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> ---
> arch/x86/lib/rwsem.S | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/lib/rwsem.S b/arch/x86/lib/rwsem.S
> index a37462a23546..02240807e97a 100644
> --- a/arch/x86/lib/rwsem.S
> +++ b/arch/x86/lib/rwsem.S
> @@ -33,10 +33,12 @@
> * value or just clobbered..
> */
>
> -#define save_common_regs \
> - pushl %ecx
> +#define save_common_regs \
> + pushl %ecx; \
> + pushl %edx
>
> -#define restore_common_regs \
> +#define restore_common_regs \
> + popl %edx; \
> popl %ecx
>
> /* Avoid uglifying the argument copying x86-64 needs to do. */
>

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