diff options
Diffstat (limited to 'arch/x86')
| -rw-r--r-- | arch/x86/kernel/entry_64.S | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 501212f14c87..eeab4cf8b2c9 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -794,6 +794,60 @@ retint_swapgs: /* return to user-space */ */ DISABLE_INTERRUPTS(CLBR_ANY) TRACE_IRQS_IRETQ + + /* + * Try to use SYSRET instead of IRET if we're returning to + * a completely clean 64-bit userspace context. + */ + movq (RCX-R11)(%rsp), %rcx + cmpq %rcx,(RIP-R11)(%rsp) /* RCX == RIP */ + jne opportunistic_sysret_failed + + /* + * On Intel CPUs, sysret with non-canonical RCX/RIP will #GP + * in kernel space. This essentially lets the user take over + * the kernel, since userspace controls RSP. It's not worth + * testing for canonicalness exactly -- this check detects any + * of the 17 high bits set, which is true for non-canonical + * or kernel addresses. (This will pessimize vsyscall=native. + * Big deal.) + * + * If virtual addresses ever become wider, this will need + * to be updated to remain correct on both old and new CPUs. + */ + .ifne __VIRTUAL_MASK_SHIFT - 47 + .error "virtual address width changed -- sysret checks need update" + .endif + shr $__VIRTUAL_MASK_SHIFT, %rcx + jnz opportunistic_sysret_failed + + cmpq $__USER_CS,(CS-R11)(%rsp) /* CS must match SYSRET */ + jne opportunistic_sysret_failed + + movq (R11-ARGOFFSET)(%rsp), %r11 + cmpq %r11,(EFLAGS-ARGOFFSET)(%rsp) /* R11 == RFLAGS */ + jne opportunistic_sysret_failed + + testq $X86_EFLAGS_RF,%r11 /* sysret can't restore RF */ + jnz opportunistic_sysret_failed + + /* nothing to check for RSP */ + + cmpq $__USER_DS,(SS-ARGOFFSET)(%rsp) /* SS must match SYSRET */ + jne opportunistic_sysret_failed + + /* + * We win! This label is here just for ease of understanding + * perf profiles. Nothing jumps here. + */ +irq_return_via_sysret: + CFI_REMEMBER_STATE + RESTORE_ARGS 1,8,1 + movq (RSP-RIP)(%rsp),%rsp + USERGS_SYSRET64 + CFI_RESTORE_STATE + +opportunistic_sysret_failed: SWAPGS jmp restore_args |