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authorAndy Lutomirski <luto@kernel.org>2015-04-01 14:26:34 -0700
committerIngo Molnar <mingo@kernel.org>2015-04-02 11:09:54 +0200
commit7ea24169097d3d3a3eab2dcc5773bc43fd5593e7 (patch)
parent80313b3078fcd2ca51970880d90757f05879a193 (diff)
x86/asm/entry/64: Disable opportunistic SYSRET if regs->flags has TF set
When I wrote the opportunistic SYSRET code, I missed an important difference between SYSRET and IRET. Both instructions are capable of setting EFLAGS.TF, but they behave differently when doing so: - IRET will not issue a #DB trap after execution when it sets TF. This is critical -- otherwise you'd never be able to make forward progress when returning to userspace. - SYSRET, on the other hand, will trap with #DB immediately after returning to CPL3, and the next instruction will never execute. This breaks anything that opportunistically SYSRETs to a user context with TF set. For example, running this code with TF set and a SIGTRAP handler loaded never gets past 'post_nop': extern unsigned char post_nop[]; asm volatile ("pushfq\n\t" "popq %%r11\n\t" "nop\n\t" "post_nop:" : : "c" (post_nop) : "r11"); In my defense, I can't find this documented in the AMD or Intel manual. Fix it by using IRET to restore TF. Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Borislav Petkov <bp@suse.de> 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: Thomas Gleixner <tglx@linutronix.de> Fixes: 2a23c6b8a9c4 ("x86_64, entry: Use sysret to return to userspace when possible") Link: http://lkml.kernel.org/r/9472f1ca4c19a38ecda45bba9c91b7168135fcfa.1427923514.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
1 files changed, 15 insertions, 1 deletions
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 2babb393915e..f0095a76c182 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -799,7 +799,21 @@ retint_swapgs: /* return to user-space */
cmpq %r11,(EFLAGS-ARGOFFSET)(%rsp) /* R11 == RFLAGS */
jne opportunistic_sysret_failed
- testq $X86_EFLAGS_RF,%r11 /* sysret can't restore RF */
+ /*
+ * SYSRET can't restore RF. 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'.
+ */
+ testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
jnz opportunistic_sysret_failed
/* nothing to check for RSP */