6.1-stable review patch. If anyone has any objections, please let me know.
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From: Brian Gerst brgerst@gmail.com
[ 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@gmail.com Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Andy Lutomirski luto@kernel.org Cc: Brian Gerst brgerst@gmail.com Cc: Denys Vlasenko dvlasenk@redhat.com Cc: H. Peter Anvin hpa@zytor.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Josh Poimboeuf jpoimboe@redhat.com Link: https://lore.kernel.org/r/20230721161018.50214-2-brgerst@gmail.com Stable-dep-of: ac8b270b61d4 ("x86/bhi: Avoid warning in #DB handler due to BHI mitigation") Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/entry/entry_64.S | 19 +++---------------- 1 file changed, 3 insertions(+), 16 deletions(-)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 6624806e6904b..a114338380a6f 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -167,22 +167,9 @@ 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