From: Thadeu Lima de Souza Cascardo cascardo@canonical.com Subject: fs/binfmt_misc.c: do not allow offset overflow

WHen registering a new binfmt_misc handler, it is possible to overflow the offset to get a negative value, which might crash the system, or possibly leak kernel data.

Here is a crash log when 2500000000 was used as an offset:

[ 6050.251552] BUG: unable to handle kernel paging request at ffff989cfd6edca0 [ 6050.252053] IP: load_misc_binary+0x22b/0x470 [binfmt_misc] [ 6050.252053] PGD 1ef3e067 P4D 1ef3e067 PUD 0 [ 6050.252053] Oops: 0000 [#1] SMP NOPTI [ 6050.252053] Modules linked in: binfmt_misc kvm_intel ppdev kvm irqbypass joydev input_leds serio_raw mac_hid parport_pc qemu_fw_cfg parpy [ 6050.252053] CPU: 0 PID: 2499 Comm: bash Not tainted 4.15.0-22-generic #24-Ubuntu [ 6050.252053] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014 [ 6050.252053] RIP: 0010:load_misc_binary+0x22b/0x470 [binfmt_misc] [ 6050.252053] RSP: 0018:ffffb6e383017e18 EFLAGS: 00010202 [ 6050.252053] RAX: 0000000000000003 RBX: ffff989d74a47100 RCX: ffff989cfd6edca0 [ 6050.252053] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff989d7d2e95e5 [ 6050.252053] RBP: ffffb6e383017e48 R08: 0000000000000001 R09: 0000000000000000 [ 6050.252053] R10: 0000000000000000 R11: fefefefefefefeff R12: 0000000000000001 [ 6050.252053] R13: ffff989d7d2e9580 R14: 0000000000000000 R15: ffffffffc0592160 [ 6050.252053] FS: 00007fa424c89740(0000) GS:ffff989d7fc00000(0000) knlGS:0000000000000000 [ 6050.252053] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 6050.252053] CR2: ffff989cfd6edca0 CR3: 000000003db08000 CR4: 00000000000006f0 [ 6050.252053] Call Trace: [ 6050.252053] search_binary_handler+0x97/0x1d0 [ 6050.252053] do_execveat_common.isra.34+0x667/0x810 [ 6050.252053] SyS_execve+0x31/0x40 [ 6050.252053] do_syscall_64+0x73/0x130 [ 6050.252053] entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Use kstrtoint instead of simple_strtoul. It will work as the code already set the delimiter byte to '\0' and we only do it when the field is not empty.

Tested with offsets -1, 2500000000, UINT_MAX and INT_MAX. Also tested with examples documented at Documentation/admin-guide/binfmt-misc.rst and other registrations from packages on Ubuntu.

Link: http://lkml.kernel.org/r/20180529135648.14254-1-cascardo@canonical.com Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/binfmt_misc.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-)

diff -puN fs/binfmt_misc.c~fs-binfmt_miscc-do-not-allow-offset-overflow fs/binfmt_misc.c --- a/fs/binfmt_misc.c~fs-binfmt_miscc-do-not-allow-offset-overflow +++ a/fs/binfmt_misc.c @@ -387,8 +387,13 @@ static Node *create_entry(const char __u s = strchr(p, del); if (!s) goto einval; - *s++ = '\0'; - e->offset = simple_strtoul(p, &p, 10); + *s = '\0'; + if (p != s) { + int r = kstrtoint(p, 10, &e->offset); + if (r != 0 || e->offset < 0) + goto einval; + } + p = s; if (*p++) goto einval; pr_debug("register: offset: %#x\n", e->offset); @@ -428,7 +433,8 @@ static Node *create_entry(const char __u if (e->mask && string_unescape_inplace(e->mask, UNESCAPE_HEX) != e->size) goto einval; - if (e->size + e->offset > BINPRM_BUF_SIZE) + if (e->size > BINPRM_BUF_SIZE || + BINPRM_BUF_SIZE - e->size < e->offset) goto einval; pr_debug("register: magic/mask length: %i\n", e->size); if (USE_DEBUG) { _