Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow
From: Borislav Petkov
Date: Tue May 11 2021 - 14:36:42 EST
On Wed, Apr 21, 2021 at 09:48:55PM -0700, Chang S. Bae wrote:
> 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.
So I did play with this more and modified
tools/testing/selftests/sigaltstack/sas.c, see diff at the end. It uses
MINSIGSTKSZ as the alt stack size and with it, sas.c does:
# [NOTE] the stack size is 2048, AT_MINSIGSTKSZ: 3632
TAP version 13
1..3
ok 1 Initial sigaltstack state was SS_DISABLE
# sstack: 0x7fdc2cbf1000, ss_size: 2048
# [NOTE] sigaltstack success
# [NOTE] Will mmap user stack
# [NOTE] Will getcontext
# [NOTE] Will makecontext
# [NOTE] Will raise SIGUSR1
Segmentation fault (core dumped)
and dmesg has:
[ 2245.641230] signal: get_sigframe: nested_altstack: 0, sp: 0x7ffe50a4d9d0, ka->sa.sa_flags: 0xc000004
[ 2245.641240] signal: get_sigframe: SA_ONSTACK, sas_ss_flags(sp): 0x0
[ 2245.641243] signal: get_sigframe: sp: 0x7fdc2cbf1800, entering_altstack
[ 2245.641245] signal: get_sigframe: nested_altstack: 0, entering_altstack: 1, __on_sig_stack: 0
Those are just debugging stuff, ignore them.
[ 2245.641249] signal: sas[8890] overflowed sigaltstack
So we do detect those overflows now.
I clumsily tried to register a SIGSEGV handler with
act.sa_sigaction = my_sigsegv;
sigaction(SIGSEGV, &act, NULL);
but that doesn't fire - task gets killed. Maybe I'm doing it wrong.
> @@ -272,8 +281,15 @@ 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",
This needs a "\n" at the end of the
string.
> + 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/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index c9c254d5791e..19eb9760e0b5 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -234,6 +234,12 @@ static unsigned long align_sigframe(unsigned long sp)
return sp;
}
+#define dbg(fmt, args...) \
+({ \
+ if (!strcmp(current->comm, "sas")) \
+ pr_err("%s: " fmt "\n", __func__, ##args); \
+})
+
static void __user *
get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
void __user **fpstate)
@@ -250,8 +256,14 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
if (IS_ENABLED(CONFIG_X86_64))
sp -= 128;
+ dbg("nested_altstack: %d, sp: 0x%lx, ka->sa.sa_flags: 0x%lx",
+ nested_altstack, sp, ka->sa.sa_flags);
+
/* This is the X/Open sanctioned signal stack switching. */
if (ka->sa.sa_flags & SA_ONSTACK) {
+
+ dbg("SA_ONSTACK, sas_ss_flags(sp): 0x%x", sas_ss_flags(sp));
+
/*
* This checks nested_altstack via sas_ss_flags(). Sensible
* programs use SS_AUTODISARM, which disables that check, and
@@ -260,6 +272,7 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
if (sas_ss_flags(sp) == 0) {
sp = current->sas_ss_sp + current->sas_ss_size;
entering_altstack = true;
+ dbg("sp: 0x%lx, entering_altstack", sp);
}
} else if (IS_ENABLED(CONFIG_X86_32) &&
!nested_altstack &&
@@ -277,6 +290,9 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
sp = align_sigframe(sp - frame_size);
+ dbg("nested_altstack: %d, entering_altstack: %d, __on_sig_stack: %d",
+ nested_altstack, entering_altstack, __on_sig_stack(sp));
+
/*
* 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.
diff --git a/tools/testing/selftests/sigaltstack/Makefile b/tools/testing/selftests/sigaltstack/Makefile
index 3e96d5d47036..b5ac8f9f0c7e 100644
--- a/tools/testing/selftests/sigaltstack/Makefile
+++ b/tools/testing/selftests/sigaltstack/Makefile
@@ -1,5 +1,5 @@
# SPDX-License-Identifier: GPL-2.0-only
-CFLAGS = -Wall
+CFLAGS = -Wall -g
TEST_GEN_PROGS = sas
include ../lib.mk
diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/sigaltstack/sas.c
index c53b070755b6..f4c4f5418a08 100644
--- a/tools/testing/selftests/sigaltstack/sas.c
+++ b/tools/testing/selftests/sigaltstack/sas.c
@@ -39,6 +39,15 @@ struct stk_data {
int flag;
};
+static char *my_strcpy(char *dest, const char *src)
+{
+ char *tmp = dest;
+
+ while ((*dest++ = *src++) != '\0')
+ /* nothing */;
+ return tmp;
+}
+
void my_usr1(int sig, siginfo_t *si, void *u)
{
char *aa;
@@ -60,7 +69,7 @@ void my_usr1(int sig, siginfo_t *si, void *u)
aa = alloca(1024);
assert(aa);
p = (struct stk_data *)(aa + 512);
- strcpy(p->msg, msg);
+ my_strcpy(p->msg, msg);
p->flag = 1;
ksft_print_msg("[RUN]\tsignal USR1\n");
err = sigaltstack(NULL, &stk);
@@ -101,6 +110,11 @@ void my_usr2(int sig, siginfo_t *si, void *u)
}
}
+void my_sigsegv(int sig, siginfo_t *si, void *u)
+{
+ ksft_print_msg("[NOTE]\tsignal SEGV\n");
+}
+
static void switch_fn(void)
{
ksft_print_msg("[RUN]\tswitched to user ctx\n");
@@ -115,18 +129,33 @@ int main(void)
int err;
/* Make sure more than the required minimum. */
- stack_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
- ksft_print_msg("[NOTE]\tthe stack size is %lu\n", stack_size);
+// stack_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;
+ stack_size = MINSIGSTKSZ;
+ ksft_print_msg("[NOTE]\tthe stack size is %lu, AT_MINSIGSTKSZ: %lu\n",
+ stack_size, getauxval(AT_MINSIGSTKSZ));
ksft_print_header();
ksft_set_plan(3);
+ /* clear signal set */
sigemptyset(&act.sa_mask);
+
+ /* a registered stack_t will be used */
act.sa_flags = SA_ONSTACK | SA_SIGINFO;
+
+ /* SA_SIGINFO means sa_sigaction specifies the signal handler */
act.sa_sigaction = my_usr1;
+
+ /* change the signal action on SIGUSR1 using @act desc */
sigaction(SIGUSR1, &act, NULL);
+
+ /* same for SIGUSR2 */
act.sa_sigaction = my_usr2;
sigaction(SIGUSR2, &act, NULL);
+
+ act.sa_sigaction = my_sigsegv;
+ sigaction(SIGSEGV, &act, NULL);
+
sstack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (sstack == MAP_FAILED) {
@@ -150,6 +179,8 @@ int main(void)
stk.ss_sp = sstack;
stk.ss_size = stack_size;
+ ksft_print_msg("sstack: 0x%lx, ss_size: %d\n", sstack, stack_size);
+
stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
err = sigaltstack(&stk, NULL);
if (err) {
@@ -169,21 +200,45 @@ int main(void)
strerror(errno));
return EXIT_FAILURE;
}
+ } else {
+ ksft_print_msg("[NOTE]\tsigaltstack success\n");
}
+ ksft_print_msg("[NOTE]\tWill mmap user stack\n");
+
ustack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (ustack == MAP_FAILED) {
ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
return EXIT_FAILURE;
}
+
+ ksft_print_msg("[NOTE]\tWill getcontext\n");
+
+ /* init @uc to the currently active context */
getcontext(&uc);
+
+ /* do not resume to other context when current context terminates */
uc.uc_link = NULL;
+
+ /* set up the stack to use */
uc.uc_stack.ss_sp = ustack;
uc.uc_stack.ss_size = stack_size;
+
+ ksft_print_msg("[NOTE]\tWill makecontext\n");
+
+ /*
+ * Run @switch_fn when this context is activated with setcontext or
+ * swapcontext.
+ */
makecontext(&uc, switch_fn, 0);
+
+ ksft_print_msg("[NOTE]\tWill raise SIGUSR1\n");
+
raise(SIGUSR1);
+ ksft_print_msg("[NOTE]\tWill sigaltstack\n");
+
err = sigaltstack(NULL, &stk);
if (err) {
ksft_exit_fail_msg("sigaltstack() - %s\n", strerror(errno));
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette