CVE-2024-50301

In the Linux kernel, the following vulnerability has been resolved:

security/keys: fix slab-out-of-bounds in key_task_permission

KASAN reports an out of bounds read: BUG: KASAN: slab-out-of-bounds in __kuid_val include/linux/uidgid.h:36 BUG: KASAN: slab-out-of-bounds in uid_eq include/linux/uidgid.h:63 [inline] BUG: KASAN: slab-out-of-bounds in key_task_permission+0x394/0x410 security/keys/permission.c:54 Read of size 4 at addr ffff88813c3ab618 by task stress-ng/4362

CPU: 2 PID: 4362 Comm: stress-ng Not tainted 5.10.0-14930-gafbffd6c3ede #15 Call Trace: __dump_stack lib/dump_stack.c:82 [inline] dump_stack+0x107/0x167 lib/dump_stack.c:123 print_address_description.constprop.0+0x19/0x170 mm/kasan/report.c:400 __kasan_report.cold+0x6c/0x84 mm/kasan/report.c:560 kasan_report+0x3a/0x50 mm/kasan/report.c:585 __kuid_val include/linux/uidgid.h:36 [inline] uid_eq include/linux/uidgid.h:63 [inline] key_task_permission+0x394/0x410 security/keys/permission.c:54 search_nested_keyrings+0x90e/0xe90 security/keys/keyring.c:793

This issue was also reported by syzbot.

It can be reproduced by following these steps(more details [1]):

1. Obtain more than 32 inputs that have similar hashes, which ends with the pattern 0xxxxxxxe6.
2. Reboot and add the keys obtained in step 1.

The reproducer demonstrates how this issue happened:

1. In the search_nested_keyrings function, when it iterates through the slots in a node(below tag ascend_to_node), if the slot pointer is meta and node->back_pointer != NULL(it means a root), it will proceed to descend_to_node. However, there is an exception. If node is the root, and one of the slots points to a shortcut, it will be treated as a keyring.

2. Whether the ptr is keyring decided by keyring_ptr_is_keyring function. However, KEYRING_PTR_SUBTYPE is 0x2UL, the same as ASSOC_ARRAY_PTR_SUBTYPE_MASK.

3. When 32 keys with the similar hashes are added to the tree, the ROOT has keys with hashes that are not similar (e.g. slot 0) and it splits NODE A without using a shortcut. When NODE A is filled with keys that all hashes are xxe6, the keys are similar, NODE A will split with a shortcut. Finally, it forms the tree as shown below, where slot 6 points to a shortcut.

                  NODE A
          +------>+---+
   

   ROOT | | 0 | xxe6 +—+ | +—+ xxxx | 0 | shortcut : : xxe6 +—+ | +—+ xxe6 : : | | | xxe6 +—+ | +—+ | 6 |—+ : : xxe6 +—+ +—+ xxe6 : : | f | xxe6 +—+ +—+ xxe6 | f | +—+

4. As mentioned above, If a slot(slot 6) of the root points to a shortcut, it may be mistakenly transferred to a key*, leading to a read out-of-bounds read.

To fix this issue, one should jump to descend_to_node if the ptr is a shortcut, regardless of whether the node is root or not.

[1] https://lore.kernel.org/linux-kernel/1cfa878e-8c7b-4570-8606-21daf5e13ce7@huaweicloud.com/

[jarkko: tweaked the commit message a bit to have an appropriate closes tag.]

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/199c20fb7499c79557a075dc24e9a7dae7d9f1ce
• https://git.kernel.org/stable/c/1e4332581cd4eed75aea77af6f66cdcdda8b49b9
• https://git.kernel.org/stable/c/3e79ad156bedf2da0ab909a118d2cec6c9c22b79
• https://git.kernel.org/stable/c/4a74da044ec9ec8679e6beccc4306b936b62873f
• https://git.kernel.org/stable/c/4efb69a0e294ef201bcdf7ce3d6202cd0a545a5d
• https://git.kernel.org/stable/c/bbad2d5b6c99db468d8f88b6ba6a56ed409b4881
• https://git.kernel.org/stable/c/c3ce634ad953ce48c75c39bdfd8b711dd95f346f
• https://git.kernel.org/stable/c/e0a317ad68e4ea48a0158187238c5407e4fdec8b