A recent refactoring of the powerpc page fault handler caused
access to protected memory regions to indicate SEGV_MAPERR instead
of the traditional SEGV_ACCERR in the si_code field of a user-space
signal handler. This can confuse debug libraries that temporarily
change the protection of memory regions, and expect to use
SEGV_ACCERR as an indication to restore access to a region.
This change restores the previous behavior. The following program
exhibits the issue:
$ ./repro read || echo "FAILED"
$ ./repro write || echo "FAILED"
$ ./repro exec || echo "FAILED"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/mman.h>
#include <assert.h>
static void segv_handler(int n, siginfo_t *info, void *arg) {
_exit(info->si_code == SEGV_ACCERR ? 0 : 1);
}
int main(int argc, char **argv)
{
void *p = NULL;
struct sigaction act = {
.sa_sigaction = segv_handler,
.sa_flags = SA_SIGINFO,
};
assert(argc == 2);
p = mmap(NULL, getpagesize(),
(strcmp(argv[1], "write") == 0) ? PROT_READ : 0,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
assert(p != MAP_FAILED);
assert(sigaction(SIGSEGV, &act, NULL) == 0);
if (strcmp(argv[1], "read") == 0)
printf("%c", *(unsigned char *)p);
else if (strcmp(argv[1], "write") == 0)
*(unsigned char *)p = 0;
else if (strcmp(argv[1], "exec") == 0)
((void (*)(void))p)();
return 1; /* failed to generate SEGV */
}
Signed-off-by: John Sperbeck <jsperb...@google.com>
---
arch/powerpc/mm/fault.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 4797d08581ce..6e1e39035380 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -145,6 +145,11 @@ static noinline int bad_area(struct pt_regs *regs,
unsigned long address)
return __bad_area(regs, address, SEGV_MAPERR);
}
+static noinline int bad_access(struct pt_regs *regs, unsigned long address)
+{
+ return __bad_area(regs, address, SEGV_ACCERR);
+}
+
static int do_sigbus(struct pt_regs *regs, unsigned long address,
unsigned int fault)
{
@@ -490,7 +495,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned
long address,
good_area:
if (unlikely(access_error(is_write, is_exec, vma)))
- return bad_area(regs, address);
+ return bad_access(regs, address);
/*
* If for any reason at all we couldn't handle the fault,
--
2.15.1.620.gb9897f4670-goog
