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. */
--
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