Re: [PATCH] security: fix integer overflow in lsm_set_self_attr() syscall

Casey Schaufler
2024-02-14T16:53:57Z

On 2/14/2024 8:05 AM, Jann Horn wrote:
> security_setselfattr() has an integer overflow bug that leads to
> out-of-bounds access when userspace provides bogus input:
> `lctx->ctx_len + sizeof(*lctx)` is checked against `lctx->len` (and,
> redundantly, also against `size`), but there are no checks on
> `lctx->ctx_len`.
> Therefore, userspace can provide an `lsm_ctx` with `->ctx_len` set to a
> value between `-sizeof(struct lsm_ctx)` and -1, and this bogus `->ctx_len`
> will then be passed to an LSM module as a buffer length, causing LSM
> modules to perform out-of-bounds accesses.
>
> The following reproducer will demonstrate this under ASAN (if AppArmor is
> loaded as an LSM):
> ```
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <stdint.h>
> #include <stdlib.h>
> #include <sys/syscall.h>
>
> struct lsm_ctx {
> uint64_t id;
> uint64_t flags;
> uint64_t len;
> uint64_t ctx_len;
> char ctx[];
> };
>
> int main(void) {
> size_t size = sizeof(struct lsm_ctx);
> struct lsm_ctx *ctx = malloc(size);
> ctx->id = 104/*LSM_ID_APPARMOR*/;
> ctx->flags = 0;
> ctx->len = size;
> ctx->ctx_len = -sizeof(struct lsm_ctx);
> syscall(
> 460/*__NR_lsm_set_self_attr*/,
> /*attr=*/ 100/*LSM_ATTR_CURRENT*/,
> /*ctx=*/ ctx,
> /*size=*/ size,
> /*flags=*/ 0
> );
> }
> ```
>
> (I'm including an ASAN splat in the patch notes sent to the list.)
>
> Fixes: a04a1198088a ("LSM: syscalls for current process attributes")
> Signed-off-by: Jann Horn <jannh@google.com>

Acked-by: Casey Schaufler <casey@schaufler-ca.com>

> ---
> ASAN splat from the reproducer:
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in do_setattr (security/apparmor/lsm.c:860)
> Read of size 1 at addr ffff888006163abf by task setselfattr/548
>
> CPU: 0 PID: 548 Comm: setselfattr Not tainted 6.8.0-rc4-00014-g7e90b5c295ec-dirty #5
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl (lib/dump_stack.c:107)
> print_report (mm/kasan/report.c:378 mm/kasan/report.c:488)
> [...]
> kasan_report (mm/kasan/report.c:603)
> [...]
> do_setattr (security/apparmor/lsm.c:860)
> [...]
> apparmor_setselfattr (security/apparmor/lsm.c:935)
> security_setselfattr (security/security.c:4038)
> __x64_sys_lsm_set_self_attr (security/lsm_syscalls.c:55)
> do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:129)
> RIP: 0033:0x7f29a170ff59
> Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 07 6f 0c 00 f7 d8 64 89 01 48
> All code
> ========
> 0: 00 c3 add %al,%bl
> 2: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1)
> 9: 00 00 00
> c: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> 11: 48 89 f8 mov %rdi,%rax
> 14: 48 89 f7 mov %rsi,%rdi
> 17: 48 89 d6 mov %rdx,%rsi
> 1a: 48 89 ca mov %rcx,%rdx
> 1d: 4d 89 c2 mov %r8,%r10
> 20: 4d 89 c8 mov %r9,%r8
> 23: 4c 8b 4c 24 08 mov 0x8(%rsp),%r9
> 28: 0f 05 syscall
> 2a:* 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax <-- trapping instruction
> 30: 73 01 jae 0x33
> 32: c3 ret
> 33: 48 8b 0d 07 6f 0c 00 mov 0xc6f07(%rip),%rcx # 0xc6f41
> 3a: f7 d8 neg %eax
> 3c: 64 89 01 mov %eax,%fs:(%rcx)
> 3f: 48 rex.W
>
> Code starting with the faulting instruction
> ===========================================
> 0: 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax
> 6: 73 01 jae 0x9
> 8: c3 ret
> 9: 48 8b 0d 07 6f 0c 00 mov 0xc6f07(%rip),%rcx # 0xc6f17
> 10: f7 d8 neg %eax
> 12: 64 89 01 mov %eax,%fs:(%rcx)
> 15: 48 rex.W
> RSP: 002b:00007ffd41c781a8 EFLAGS: 00000202 ORIG_RAX: 00000000000001cc
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f29a170ff59
> RDX: 0000000000000020 RSI: 000056518c581260 RDI: 0000000000000064
> RBP: 00007ffd41c781c0 R08: 00000000000a3330 R09: 000056518c581260
> R10: 0000000000000000 R11: 0000000000000202 R12: 000056518bd95060
> R13: 00007ffd41c782a0 R14: 0000000000000000 R15: 0000000000000000
> </TASK>
>
> Allocated by task 548 on cpu 0 at 61.045304s:
> kasan_save_stack (mm/kasan/common.c:48)
> kasan_save_track (mm/kasan/common.c:68)
> __kasan_kmalloc (mm/kasan/common.c:372 mm/kasan/common.c:389)
> __kmalloc (./include/linux/kasan.h:211 mm/slub.c:3981 mm/slub.c:3994)
> load_elf_binary (./include/linux/slab.h:594 fs/binfmt_elf.c:880)
> bprm_execve (fs/exec.c:1783 fs/exec.c:1825 fs/exec.c:1877 fs/exec.c:1853)
> do_execveat_common.isra.0 (fs/exec.c:1984)
> __x64_sys_execve (fs/exec.c:2129 (discriminator 1))
> do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:129)
>
> Freed by task 548 on cpu 0 at 61.045380s:
> kasan_save_stack (mm/kasan/common.c:48)
> kasan_save_track (mm/kasan/common.c:68)
> kasan_save_free_info (mm/kasan/generic.c:643 (discriminator 1))
> poison_slab_object (mm/kasan/common.c:243)
> __kasan_slab_free (mm/kasan/common.c:257 (discriminator 1))
> kfree (mm/slub.c:4299 (discriminator 3) mm/slub.c:4409 (discriminator 3))
> load_elf_binary (fs/binfmt_elf.c:896 (discriminator 1))
> bprm_execve (fs/exec.c:1783 fs/exec.c:1825 fs/exec.c:1877 fs/exec.c:1853)
> do_execveat_common.isra.0 (fs/exec.c:1984)
> __x64_sys_execve (fs/exec.c:2129 (discriminator 1))
> do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:129)
>
> The buggy address belongs to the object at ffff888006163a80
> which belongs to the cache kmalloc-32 of size 32
> The buggy address is located 31 bytes to the right of
> allocated 32-byte region [ffff888006163a80, ffff888006163aa0)
>
> The buggy address belongs to the physical page:
> page:0000000021a8da3a refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x6163
> flags: 0x100000000000800(slab|node=0|zone=1)
> page_type: 0xffffffff()
> raw: 0100000000000800 ffff888001042500 dead000000000122 0000000000000000
> raw: 0000000000000000 0000000080400040 00000001ffffffff 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
> ffff888006163980: fa fb fb fb fc fc fc fc 00 00 00 00 fc fc fc fc
> ffff888006163a00: fa fb fb fb fc fc fc fc fa fb fb fb fc fc fc fc
>> ffff888006163a80: fa fb fb fb fc fc fc fc 00 00 00 00 fc fc fc fc
> ^
> ffff888006163b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff888006163b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
>
> security/security.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/security/security.c b/security/security.c
> index 3aaad75c9ce8..7035ee35a393 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -29,6 +29,7 @@
> #include <linux/backing-dev.h>
> #include <linux/string.h>
> #include <linux/msg.h>
> +#include <linux/overflow.h>
> #include <net/flow.h>
>
> /* How many LSMs were built into the kernel? */
> @@ -4015,6 +4016,7 @@ int security_setselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
> struct security_hook_list *hp;
> struct lsm_ctx *lctx;
> int rc = LSM_RET_DEFAULT(setselfattr);
> + u64 required_len;
>
> if (flags)
> return -EINVAL;
> @@ -4027,8 +4029,9 @@ int security_setselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
> if (IS_ERR(lctx))
> return PTR_ERR(lctx);
>
> - if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
> - lctx->len < lctx->ctx_len + sizeof(*lctx)) {
> + if (size < lctx->len ||
> + check_add_overflow(sizeof(*lctx), lctx->ctx_len, &required_len) ||
> + lctx->len < required_len) {
> rc = -EINVAL;
> goto free_out;
> }
>
> base-commit: 7e90b5c295ec1e47c8ad865429f046970c549a66