From: Chang S. Bae chang.seok.bae@intel.com
[ Upstream commit 2beb4a53fc3f1081cedc1c1a198c7f56cc4fc60c ]
The kernel pushes context on to the userspace stack to prepare for the user's signal handler. When the user has supplied an alternate signal stack, via sigaltstack(2), it is easy for the kernel to verify that the stack size is sufficient for the current hardware context.
Check if writing the hardware context to the alternate stack will exceed it's size. If yes, then instead of corrupting user-data and proceeding with the original signal handler, an immediate SIGSEGV signal is delivered.
Refactor the stack pointer check code from on_sig_stack() and use the new helper.
While the kernel allows new source code to discover and use a sufficient alternate signal stack size, this check is still necessary to protect binaries with insufficient alternate signal stack size from data corruption.
Fixes: c2bc11f10a39 ("x86, AVX-512: Enable AVX-512 States Context Switch") Reported-by: Florian Weimer fweimer@redhat.com Suggested-by: Jann Horn jannh@google.com Suggested-by: Andy Lutomirski luto@kernel.org Signed-off-by: Chang S. Bae chang.seok.bae@intel.com Signed-off-by: Borislav Petkov bp@suse.de Reviewed-by: Len Brown len.brown@intel.com Acked-by: Thomas Gleixner tglx@linutronix.de Link: https://lkml.kernel.org/r/20210518200320.17239-6-chang.seok.bae@intel.com Link: https://bugzilla.kernel.org/show_bug.cgi?id=153531 Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kernel/signal.c | 24 ++++++++++++++++++++---- include/linux/sched/signal.h | 19 ++++++++++++------- 2 files changed, 32 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index 2fdbf5ef8c39..026ce06a24c0 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -241,10 +241,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, void __user **fpstate) { /* Default to using normal stack */ + bool nested_altstack = on_sig_stack(regs->sp); + bool entering_altstack = false; unsigned long math_size = 0; unsigned long sp = regs->sp; unsigned long buf_fx = 0; - int onsigstack = on_sig_stack(sp); int ret;
/* redzone */ @@ -253,15 +254,23 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
/* This is the X/Open sanctioned signal stack switching. */ if (ka->sa.sa_flags & SA_ONSTACK) { - if (sas_ss_flags(sp) == 0) + /* + * This checks nested_altstack via sas_ss_flags(). Sensible + * programs use SS_AUTODISARM, which disables that check, and + * programs that don't use SS_AUTODISARM get compatible. + */ + if (sas_ss_flags(sp) == 0) { sp = current->sas_ss_sp + current->sas_ss_size; + entering_altstack = true; + } } else if (IS_ENABLED(CONFIG_X86_32) && - !onsigstack && + !nested_altstack && regs->ss != __USER_DS && !(ka->sa.sa_flags & SA_RESTORER) && ka->sa.sa_restorer) { /* This is the legacy signal stack switching. */ sp = (unsigned long) ka->sa.sa_restorer; + entering_altstack = true; }
sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32), @@ -274,8 +283,15 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, * If we are on the alternate signal stack and would overflow it, don't. * Return an always-bogus address instead so we will die with SIGSEGV. */ - if (onsigstack && !likely(on_sig_stack(sp))) + if (unlikely((nested_altstack || entering_altstack) && + !__on_sig_stack(sp))) { + + if (show_unhandled_signals && printk_ratelimit()) + pr_info("%s[%d] overflowed sigaltstack\n", + current->comm, task_pid_nr(current)); + return (void __user *)-1L; + }
/* save i387 and extended state */ ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size); diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index baf58f4cb057..b3f88470cbb5 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -509,6 +509,17 @@ static inline int kill_cad_pid(int sig, int priv) #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0) #define SEND_SIG_PRIV ((struct kernel_siginfo *) 1)
+static inline int __on_sig_stack(unsigned long sp) +{ +#ifdef CONFIG_STACK_GROWSUP + return sp >= current->sas_ss_sp && + sp - current->sas_ss_sp < current->sas_ss_size; +#else + return sp > current->sas_ss_sp && + sp - current->sas_ss_sp <= current->sas_ss_size; +#endif +} + /* * True if we are on the alternate signal stack. */ @@ -526,13 +537,7 @@ static inline int on_sig_stack(unsigned long sp) 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; -#else - return sp > current->sas_ss_sp && - sp - current->sas_ss_sp <= current->sas_ss_size; -#endif + return __on_sig_stack(sp); }
static inline int sas_ss_flags(unsigned long sp)