On Tue, 29 May 2018 10:56:48 -0300 Thadeu Lima de Souza Cascardo cascardo@canonical.com wrote:
It's possible to overflow the offset to get a negative value, which might crash the system, or possibly leak kernel data.
I think the missing information here is "when registering a new binfmt_misc binary type", yes?
Here is a crash log when using 2500000000 as 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.
Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org
Registering a handler is a priveleged operation. As such, I don't think a -stable backport is needed?