[PATCH] Fix x86_64 TIF_SYSCALL_TRACE race in entry.S

[Mathieu Desnoyers]

Fix x86_64 TIF_SYSCALL_TRACE race in entry.S

When the flag is inactive upon syscall entry and concurrently activated before
exit, we seem to reach a state where the top of stack is incorrect upon return
to user space.

Fix this by fixing the top of stack and jumping to int_ret_from_sys_call if we
detect that thread flags has been modified.

We make sure that the thread flag read is coherent between our new test and the ALLWORK_MASK test by first saving it in a register used for both comparisons.

It applies on top of 2.6.23-mm1 or 2.6.23.1. If you think the
implementation is correct, I'll port it to 2.6.24-rc1.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: Andi Kleen <ak@xxxxxx>
---
 arch/x86_64/kernel/entry.S |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Index: linux-2.6-lttng/arch/x86_64/kernel/entry.S
===================================================================
--- linux-2.6-lttng.orig/arch/x86_64/kernel/entry.S	2007-10-27 14:01:12.000000000 -0400
+++ linux-2.6-lttng/arch/x86_64/kernel/entry.S	2007-10-27 14:01:28.000000000 -0400
@@ -245,9 +245,11 @@ ret_from_sys_call:
 	/* edi:	flagmask */
 sysret_check:		
 	GET_THREAD_INFO(%rcx)
+	movl threadinfo_flags(%rcx),%edx
+	testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP),%edx
+	jnz ret_from_sys_call_trace
 	cli
 	TRACE_IRQS_OFF
-	movl threadinfo_flags(%rcx),%edx
 	andl %edi,%edx
 	jnz  sysret_careful 
 	CFI_REMEMBER_STATE
@@ -278,6 +280,14 @@ sysret_careful:
 	CFI_ADJUST_CFA_OFFSET -8
 	jmp sysret_check
 
+ret_from_sys_call_trace:
+	SAVE_REST
+	FIXUP_TOP_OF_STACK %rdi
+	movq %rsp,%rdi
+	LOAD_ARGS ARGOFFSET  /* reload args from stack in case ptrace changed it */
+	RESTORE_REST
+	jmp int_ret_from_sys_call
+
 	/* Handle a signal */ 
 sysret_signal:
 	TRACE_IRQS_ON
-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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