diff options
Diffstat (limited to 'arch/x86/entry/common.c')
| -rw-r--r-- | arch/x86/entry/common.c | 43 |
1 files changed, 42 insertions, 1 deletions
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 0551bcb197fb..90214652ff06 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -71,7 +71,8 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr) return false; } -__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr) +/* Returns true to return using SYSRET, or false to use IRET */ +__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr) { add_random_kstack_offset(); nr = syscall_enter_from_user_mode(regs, nr); @@ -85,6 +86,46 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr) instrumentation_end(); syscall_exit_to_user_mode(regs); + + /* + * Check that the register state is valid for using SYSRET to exit + * to userspace. Otherwise use the slower but fully capable IRET + * exit path. + */ + + /* XEN PV guests always use the IRET path */ + if (cpu_feature_enabled(X86_FEATURE_XENPV)) + return false; + + /* SYSRET requires RCX == RIP and R11 == EFLAGS */ + if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags)) + return false; + + /* CS and SS must match the values set in MSR_STAR */ + if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS)) + return false; + + /* + * 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. + * + * Change top bits to match the most significant bit (47th or 56th bit + * depending on paging mode) in the address. + */ + if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHIFT + 1))) + return false; + + /* + * SYSRET cannot restore RF. It can restore TF, but unlike IRET, + * restoring TF results in a trap from userspace immediately after + * SYSRET. + */ + if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF))) + return false; + + /* Use SYSRET to exit to userspace */ + return true; } #endif |