Re: [PATCH] x86/rwsem: Save and restore all callee-clobbered regs in 32-bit ____down_write()

[Guenter Roeck]

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@xxxxxxx>
>
> ____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@xxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>

Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>

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. */