If a signal stack is set up with SS_AUTODISARM, then the kernel inherently avoids incorrectly resetting the signal stack if signals recurse: the signal stack will be reset on the first signal delivery. This means that we don't need check the stack pointer when delivering signals if SS_AUTODISARM is set.
This will make segmented x86 programs more robust: currently there's a hole that could be triggered if ESP/RSP appears to point to the signal stack but actually doesn't due to a nonzero SS base. Signed-off-by: Stas Sergeev <[email protected]> Cc: Al Viro <[email protected]> Cc: Aleksa Sarai <[email protected]> Cc: Amanieu d'Antras <[email protected]> Cc: Andrea Arcangeli <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Brian Gerst <[email protected]> Cc: Denys Vlasenko <[email protected]> Cc: Eric W. Biederman <[email protected]> Cc: Frederic Weisbecker <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Heinrich Schuchardt <[email protected]> Cc: Jason Low <[email protected]> Cc: Josh Triplett <[email protected]> Cc: Konstantin Khlebnikov <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Oleg Nesterov <[email protected]> Cc: Palmer Dabbelt <[email protected]> Cc: Paul Moore <[email protected]> Cc: Pavel Emelyanov <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Richard Weinberger <[email protected]> Cc: Sasha Levin <[email protected]> Cc: Shuah Khan <[email protected]> Cc: Tejun Heo <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Vladimir Davydov <[email protected]> Cc: [email protected] Cc: [email protected] Signed-off-by: Andy Lutomirski <[email protected]> --- include/linux/sched.h | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/include/linux/sched.h b/include/linux/sched.h index 2950c5cd3005..8f03a93348b9 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2576,6 +2576,18 @@ static inline int kill_cad_pid(int sig, int priv) */ static inline int on_sig_stack(unsigned long sp) { + /* + * If the signal stack is AUTODISARM then, by construction, we + * can't be on the signal stack unless user code deliberately set + * SS_AUTODISARM when we were already on the it. + * + * This improve reliability: if user state gets corrupted such that + * the stack pointer points very close to the end of the signal stack, + * then this check will enable the signal to be handled anyway. + */ + if (current->sas_ss_flags & SS_AUTODISARM) + return 0; + #ifdef CONFIG_STACK_GROWSUP return sp >= current->sas_ss_sp && sp - current->sas_ss_sp < current->sas_ss_size; -- 2.5.5
