[PATCH] powerpc/mm: restore SEGV_ACCERR for segv on mapped region
Benjamin Herrenschmidt
benh at kernel.crashing.org
Tue Jan 2 07:55:51 AEDT 2018
On Sun, 2017-12-31 at 21:24 -0800, John Sperbeck wrote:
> 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:
Good catch !
Acked-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
>
> $ ./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 <jsperbeck at 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,
More information about the Linuxppc-dev
mailing list