This is a note to let you know that I've just added the patch titled
x86/entry/64: Remove obsolete comment on tracing vs. SYSRET
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-entry-64-remove-obsolete-comment-on-tracing-vs.-.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit c7b2984127a226ee68d8bad2c7a8e07bf2af368c
Author: Brian Gerst <brgerst@xxxxxxxxx>
Date: Fri Jul 21 12:10:12 2023 -0400
x86/entry/64: Remove obsolete comment on tracing vs. SYSRET
[ Upstream commit eb43c9b1517b48e2ff0d3a584aca197338987d7b ]
This comment comes from a time when the kernel attempted to use SYSRET
on all returns to userspace, including interrupts and exceptions. Ever
since commit fffbb5dc ("Move opportunistic sysret code to syscall code
path"), SYSRET is only used for returning from system calls. The
specific tracing issue listed in this comment is not possible anymore.
Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230721161018.50214-2-brgerst@xxxxxxxxx
Stable-dep-of: ac8b270b61d4 ("x86/bhi: Avoid warning in #DB handler due to BHI mitigation")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 10d6888713d8b..f656c6e0e4588 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -165,22 +165,9 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
jne swapgs_restore_regs_and_return_to_usermode
/*
- * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
- * restore RF properly. If the slowpath sets it for whatever reason, we
- * need to restore it correctly.
- *
- * SYSRET can restore TF, but unlike IRET, restoring TF results in a
- * trap from userspace immediately after SYSRET. This would cause an
- * infinite loop whenever #DB happens with register state that satisfies
- * the opportunistic SYSRET conditions. For example, single-stepping
- * this user code:
- *
- * movq $stuck_here, %rcx
- * pushfq
- * popq %r11
- * stuck_here:
- *
- * would never get past 'stuck_here'.
+ * SYSRET cannot restore RF. It can restore TF, but unlike IRET,
+ * restoring TF results in a trap from userspace immediately after
+ * SYSRET.
*/
testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
jnz swapgs_restore_regs_and_return_to_usermode
