commit 30bfa7b3488bfb1bb75c9f50a5fcac1832970c60
author Andy Lutomirski <luto@kernel.org> Wed Dec 16 23:18:48 2015 -0800
committer Thomas Gleixner <tglx@linutronix.de> Mon Dec 21 16:05:01 2015 +0100
tree af1e7d3bb55f826b936c7a292b8f0f389b641222
parent 6a613ac6bc015f8ef75806d397d69dbac4a8d8c4

x86/entry: Restore traditional SYSENTER calling convention

It turns out that some Android versions hardcode the SYSENTER
calling convention.  This is buggy and will cause problems no
matter what the kernel does.  Nonetheless, we should try to
support it.

Credit goes to Linus for pointing out a clean way to handle
the SYSENTER/SYSCALL clobber differences while preserving
straightforward DWARF annotations.

I believe that the original offending Android commit was:

https://android.googlesource.com/platform%2Fbionic/+/7dc3684d7a2587e43e6d2a8e0e3f39bf759bd535

Reported-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-and-tested-by: Borislav Petkov <bp@alien8.de>
Cc: <mark.gross@intel.com>
Cc: Su Tao <tao.su@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: <frank.wang@intel.com>
Cc: <borun.fu@intel.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Mingwei Shi <mingwei.shi@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/entry/common.c

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index a89fdbc..0366374 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -421,7 +421,7 @@
 	regs->ip = landing_pad;
 
 	/*
-	 * Fetch ECX from where the vDSO stashed it.
+	 * Fetch EBP from where the vDSO stashed it.
 	 *
 	 * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
 	 */
@@ -432,10 +432,10 @@
 		 * Micro-optimization: the pointer we're following is explicitly
 		 * 32 bits, so it can't be out of range.
 		 */
-		__get_user(*(u32 *)&regs->cx,
+		__get_user(*(u32 *)&regs->bp,
 			    (u32 __user __force *)(unsigned long)(u32)regs->sp)
 #else
-		get_user(*(u32 *)&regs->cx,
+		get_user(*(u32 *)&regs->bp,
 			 (u32 __user __force *)(unsigned long)(u32)regs->sp)
 #endif
 		) {