Linux Kernel PowerPC KASAN Shadow Page Update Bug CVE-2021-47640
CVE-2021-47640 Published on February 26, 2025
powerpc/kasan: Fix early region not updated correctly
In the Linux kernel, the following vulnerability has been resolved:
powerpc/kasan: Fix early region not updated correctly
The shadow's page table is not updated when PTE_RPN_SHIFT is 24
and PAGE_SHIFT is 12. It not only causes false positives but
also false negative as shown the following text.
Fix it by bringing the logic of kasan_early_shadow_page_entry here.
1. False Positive:
==================================================================
BUG: KASAN: vmalloc-out-of-bounds in pcpu_alloc+0x508/0xa50
Write of size 16 at addr f57f3be0 by task swapper/0/1
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.15.0-12267-gdebe436e77c7 #1
Call Trace:
[c80d1c20] [c07fe7b8] dump_stack_lvl+0x4c/0x6c (unreliable)
[c80d1c40] [c02ff668] print_address_description.constprop.0+0x88/0x300
[c80d1c70] [c02ff45c] kasan_report+0x1ec/0x200
[c80d1cb0] [c0300b20] kasan_check_range+0x160/0x2f0
[c80d1cc0] [c03018a4] memset+0x34/0x90
[c80d1ce0] [c0280108] pcpu_alloc+0x508/0xa50
[c80d1d40] [c02fd7bc] __kmem_cache_create+0xfc/0x570
[c80d1d70] [c0283d64] kmem_cache_create_usercopy+0x274/0x3e0
[c80d1db0] [c2036580] init_sd+0xc4/0x1d0
[c80d1de0] [c00044a0] do_one_initcall+0xc0/0x33c
[c80d1eb0] [c2001624] kernel_init_freeable+0x2c8/0x384
[c80d1ef0] [c0004b14] kernel_init+0x24/0x170
[c80d1f10] [c001b26c] ret_from_kernel_thread+0x5c/0x64
Memory state around the buggy address:
f57f3a80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
f57f3b00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>f57f3b80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
^
f57f3c00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
f57f3c80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
==================================================================
2. False Negative (with KASAN tests):
==================================================================
Before fix:
ok 45 - kmalloc_double_kzfree
# vmalloc_oob: EXPECTATION FAILED at lib/test_kasan.c:1039
KASAN failure expected in "((volatile char *)area)[3100]", but none occurred
not ok 46 - vmalloc_oob
not ok 1 - kasan
==================================================================
After fix:
ok 1 - kasan
Vulnerability Analysis
CVE-2021-47640 is exploitable with local system access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to be very high.
Weakness Type
What is a Memory Corruption Vulnerability?
The software writes data past the end, or before the beginning, of the intended buffer. Typically, this can result in corruption of data, a crash, or code execution. The software may modify an index or perform pointer arithmetic that references a memory location that is outside of the boundaries of the buffer. A subsequent write operation then produces undefined or unexpected results.
CVE-2021-47640 has been classified to as a Memory Corruption vulnerability or weakness.
Products Associated with CVE-2021-47640
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Affected Versions
Linux:- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below 7f19245c3647afea8c7c41f795506ef70f64b9f2 is affected.
- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below f39a3309393a4a484532f6ba745c6acbcfe06115 is affected.
- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below 5a3d8f3192a409893c57808cc935e16484df1068 is affected.
- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below de56beace6648065d404cd9835aa7d30e3df519d is affected.
- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below e3d157a4b4f4e0268c98be5b7013bf4b31234bb6 is affected.
- Version cbd18991e24fea2c31da3bb117c83e4a3538cd11 and below dd75080aa8409ce10d50fb58981c6b59bf8707d3 is affected.
- Version 3822dd8c102d11ada9d9ed8e04cad0b347a04689 is affected.
- Version 5.3.6 and below 5.4 is affected.
- Version 5.4 is affected.
- Before 5.4 is unaffected.
- Version 5.4.189, <= 5.4.* is unaffected.
- Version 5.10.110, <= 5.10.* is unaffected.
- Version 5.15.33, <= 5.15.* is unaffected.
- Version 5.16.19, <= 5.16.* is unaffected.
- Version 5.17.2, <= 5.17.* is unaffected.
- Version 5.18, <= * is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.