diff options
| author | Brian Gerst <brgerst@gmail.com> | 2023-10-12 01:43:49 +0300 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2023-10-13 14:05:28 +0300 |
| commit | ca282b486a570a0bfda5c1a4595ace7fa14243bf (patch) | |
| tree | 9452c4d64bcd7fdb5df5e9de4c838f14b1e67fdf /arch/x86/entry/common.c | |
| parent | bab9fa6dc5e4483749838877deebe038de3ce97e (diff) | |
| download | linux-ca282b486a570a0bfda5c1a4595ace7fa14243bf.tar.xz | |
x86/entry/64: Convert SYSRET validation tests to C
No change in functionality expected.
Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20231011224351.130935-2-brgerst@gmail.com
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 |