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Because keyctl_read_key() looks up the key with no permissions
requested, it may find a negatively instantiated key. If the key is
also possessed, we went ahead and called ->read() on the key. But the
key payload will actually contain the ->reject_error rather than the
normal payload. Thus, the kernel oopses trying to read the
user_key_payload from memory address (int)-ENOKEY = 0x00000000ffffff82.
Fortunately the payload data is stored inline, so it shouldn't be
possible to abuse this as an arbitrary memory read primitive...
Reproducer:
keyctl new_session
keyctl request2 user desc '' @s
keyctl read $(keyctl show | awk '/user: desc/ {print $1}')
It causes a crash like the following:
BUG: unable to handle kernel paging request at 00000000ffffff92
IP: user_read+0x33/0xa0
PGD 36a54067 P4D 36a54067 PUD 0
Oops: 0000 [#1] SMP
CPU: 0 PID: 211 Comm: keyctl Not tainted 4.14.0-rc1 #337
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-20170228_101828-anatol 04/01/2014
task: ffff90aa3b74c3c0 task.stack: ffff9878c0478000
RIP: 0010:user_read+0x33/0xa0
RSP: 0018:ffff9878c047bee8 EFLAGS: 00010246
RAX: 0000000000000001 RBX: ffff90aa3d7da340 RCX: 0000000000000017
RDX: 0000000000000000 RSI: 00000000ffffff82 RDI: ffff90aa3d7da340
RBP: ffff9878c047bf00 R08: 00000024f95da94f R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f58ece69740(0000) GS:ffff90aa3e200000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000ffffff92 CR3: 0000000036adc001 CR4: 00000000003606f0
Call Trace:
keyctl_read_key+0xac/0xe0
SyS_keyctl+0x99/0x120
entry_SYSCALL_64_fastpath+0x1f/0xbe
RIP: 0033:0x7f58ec787bb9
RSP: 002b:00007ffc8d401678 EFLAGS: 00000206 ORIG_RAX: 00000000000000fa
RAX: ffffffffffffffda RBX: 00007ffc8d402800 RCX: 00007f58ec787bb9
RDX: 0000000000000000 RSI: 00000000174a63ac RDI: 000000000000000b
RBP: 0000000000000004 R08: 00007ffc8d402809 R09: 0000000000000020
R10: 0000000000000000 R11: 0000000000000206 R12: 00007ffc8d402800
R13: 00007ffc8d4016e0 R14: 0000000000000000 R15: 0000000000000000
Code: e5 41 55 49 89 f5 41 54 49 89 d4 53 48 89 fb e8 a4 b4 ad ff 85 c0 74 09 80 3d b9 4c 96 00 00 74 43 48 8b b3 20 01 00 00 4d 85 ed <0f> b7 5e 10 74 29 4d 85 e4 74 24 4c 39 e3 4c 89 e2 4c 89 ef 48
RIP: user_read+0x33/0xa0 RSP: ffff9878c047bee8
CR2: 00000000ffffff92
Fixes: 61ea0c0ba9 ("KEYS: Skip key state checks when checking for possession")
Cc: <stable@vger.kernel.org> [v3.13+]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.