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0ebd37a222
If a process uses alternative signal stack by using sigaltstack(),
then that stack overflows and stack wraparound occurs.
Simple Explanation:
The accurate sp order is A,B,C,D,...
But now the sp points to A,B,C and A,B,C again.
This problem can reproduce by the following code:
$ cat test_sigaltstack.c
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
volatile int counter = 0;
void print_sp()
{
unsigned long sp;
__asm__ __volatile__("move %0, $sp" : "=r" (sp));
printf("sp = 0x%08lx\n", sp);
}
void segv_handler()
{
int *c = NULL;
print_sp();
counter++;
printf("%d\n", counter);
if (counter == 23)
abort();
*c = 1; // SEGV
}
int main()
{
int *c = NULL;
char *s = malloc(SIGSTKSZ);
stack_t stack;
struct sigaction action;
memset(s, 0, SIGSTKSZ);
stack.ss_sp = s;
stack.ss_flags = 0;
stack.ss_size = SIGSTKSZ;
if (sigaltstack(&stack, NULL)) {
printf("Failed to use sigaltstack!\n");
return -1;
}
memset(&action, 0, sizeof(action));
action.sa_handler = segv_handler;
action.sa_flags = SA_ONSTACK | SA_NODEFER;
sigemptyset(&action.sa_mask);
sigaction(SIGSEGV, &action, NULL);
*c = 0; //SEGV
if (!s)
free(s);
return 0;
}
$ gcc test_sigaltstack.c -o test_sigaltstack
$ ./test_sigaltstack
sp = 0x120015c80
1
sp = 0x120015900
2
sp = 0x120015580
3
sp = 0x120015200
4
sp = 0x120014e80
5
sp = 0x120014b00
6
sp = 0x120014780
7
sp = 0x120014400
8
sp = 0x120014080
9
sp = 0x120013d00
10
sp = 0x120015c80
11 # wraparound occurs! the 11nd output is same as 1st.
sp = 0x120015900
12
sp = 0x120015580
13
sp = 0x120015200
14
sp = 0x120014e80
15
sp = 0x120014b00
16
sp = 0x120014780
17
sp = 0x120014400
18
sp = 0x120014080
19
sp = 0x120013d00
20
sp = 0x120015c80
21 # wraparound occurs! the 21nd output is same as 1st.
sp = 0x120015900
22
sp = 0x120015580
23
Aborted
With this patch:
$ ./test_sigaltstack
sp = 0x120015c80
1
sp = 0x120015900
2
sp = 0x120015580
3
sp = 0x120015200
4
sp = 0x120014e80
5
sp = 0x120014b00
6
sp = 0x120014780
7
sp = 0x120014400
8
sp = 0x120014080
9
Segmentation fault
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.
This patch is similar with commit 83bd01024b ("x86: protect against
sigaltstack wraparound").
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Linux kernel
============
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