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SubjectRe: [PATCH] swapfile: fix memory corruption via malformed swapfile
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On 10/31/2016 10:32 PM, Jann Horn wrote:
> When root activates a swap partition whose header has the wrong endianness,
> nr_badpages elements of badpages are swabbed before nr_badpages has been
> checked, leading to a buffer overrun of up to 8GB.
>
> This normally is not a security issue because it can only be exploited by
> root (more specifically, a process with CAP_SYS_ADMIN or the ability to
> modify a swap file/partition), and such a process can already e.g. modify
> swapped-out memory of any other userspace process on the system.
>
> Testcase for reproducing the bug (must be run as root, should crash your
> kernel):
> =================
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/swap.h>
> #include <limits.h>
> #include <err.h>
> #include <string.h>
> #include <stdio.h>
>
> #define PAGE_SIZE 4096
> #define __u32 unsigned int
>
>
> // from include/linux/swap.h
> union swap_header {
> struct {
> char reserved[PAGE_SIZE - 10];
> char magic[10]; /* SWAP-SPACE or SWAPSPACE2 */
> } magic;
> struct {
> char bootbits[1024]; /* Space for disklabel etc. */
> __u32 version;
> __u32 last_page;
> __u32 nr_badpages;
> unsigned char sws_uuid[16];
> unsigned char sws_volume[16];
> __u32 padding[117];
> __u32 badpages[1];
> } info;
> };
>
> int main(void) {
> char file[] = "/tmp/swapfile.XXXXXX";
> int file_fd = mkstemp(file);
> if (file_fd == -1)
> err(1, "mkstemp");
> if (ftruncate(file_fd, PAGE_SIZE))
> err(1, "ftruncate");
> union swap_header swap_header = {
> .info = {
> .version = __builtin_bswap32(1),
> .nr_badpages = __builtin_bswap32(INT_MAX)
> }
> };
> memcpy(swap_header.magic.magic, "SWAPSPACE2", 10);
> if (write(file_fd, &swap_header, sizeof(swap_header)) !=
> sizeof(swap_header))
> err(1, "write");
>
> // not because the attack needs it, just in case you forgot to
> // sync yourself before crashing your machine
> sync();
>
> // now die
> if (swapon(file, 0))
> err(1, "swapon");
> puts("huh, we survived");
> if (swapoff(file))
> err(1, "swapoff");
> unlink(file);
> }
> =================
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jann@thejh.net>
> ---
> mm/swapfile.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2210de290b54..f30438970cd1 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2224,6 +2224,8 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
> swab32s(&swap_header->info.version);
> swab32s(&swap_header->info.last_page);
> swab32s(&swap_header->info.nr_badpages);
> + if (swap_header->info.nr_badpages > MAX_SWAP_BADPAGES)
> + return 0;
> for (i = 0; i < swap_header->info.nr_badpages; i++)
> swab32s(&swap_header->info.badpages[i]);
> }
>

Nice catch!

Acked-by: Jerome Marchand <jmarchan@redhat.com>



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