The following commit has been merged into the x86/iopl branch of tip:
Commit-ID: a3ba966066afbe8fd0d3605ffe04c633083752f1
Gitweb: https://git.kernel.org/tip/a3ba966066afbe8fd0d3605ffe04c633083752f1
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Sat, 16 Nov 2019 11:12:03 +01:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 16 Nov 2019 11:24:07 +01:00
x86/entry/32: Clarify register saving in __switch_to_asm()
commit 6690e86be83a ("sched/x86: Save [ER]FLAGS on context switch")
re-introduced the flags saving on context switch to prevent AC leakage.
The pushf/popf instructions are right among the callee saved register
section, so the comment explaining the save/restore is not entirely
correct.
Add a seperate comment to pushf/popf explaining the reason.
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
arch/x86/entry/entry_32.S | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index f83ca5a..99fad6f 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -718,6 +718,11 @@ ENTRY(__switch_to_asm)
pushl %ebx
pushl %edi
pushl %esi
+ /*
+ * Flags are saved to prevent AC leakage. This could go
+ * away if objtool would have 32bit support to verify
+ * the STAC/CLAC correctness.
+ */
pushfl
/* switch stack */
@@ -740,8 +745,9 @@ ENTRY(__switch_to_asm)
FILL_RETURN_BUFFER %ebx, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
#endif
- /* restore callee-saved registers */
+ /* Restore flags or the incoming task to restore AC state. */
popfl
+ /* restore callee-saved registers */
popl %esi
popl %edi
popl %ebx
|