CVE-2024-50301: security/keys: fix slab-out-of-bounds in key_task_permission

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

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

KASAN reports an out of bounds read: BUG: KASAN: slab-out-of-bounds in kuidval include/linux/uidgid.h:36 BUG: KASAN: slab-out-of-bounds in uideq include/linux/uidgid.h:63 [inline] BUG: KASAN: slab-out-of-bounds in keytaskpermission+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: dumpstack lib/dumpstack.c:82 [inline] dumpstack+0x107/0x167 lib/dumpstack.c:123 printaddressdescription.constprop.0+0x19/0x170 mm/kasan/report.c:400 kasanreport.cold+0x6c/0x84 mm/kasan/report.c:560 kasanreport+0x3a/0x50 mm/kasan/report.c:585 kuidval include/linux/uidgid.h:36 [inline] uideq include/linux/uidgid.h:63 [inline] keytaskpermission+0x394/0x410 security/keys/permission.c:54 searchnestedkeyrings+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 searchnestedkeyrings function, when it iterates through the slots in a node(below tag ascendtonode), if the slot pointer is meta and node->backpointer != NULL(it means a root), it will proceed to descendtonode. 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 keyringptriskeyring function. However, KEYRINGPTRSUBTYPE is 0x2UL, the same as ASSOCARRAYPTRSUBTYPEMASK. 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 descendtonode 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.]