Re: CVE-2021-46966: ACPI: custom_method: fix potential use-after-free issue

[Michal Hocko]

Hi,
this seems like another example of a reasonable fix with a very dubious
CVE IMHO. Allowing access to /sys/kernel/debug/acpi/custom_method to
anybody but trusted actor is a huge security problem on its own. I
really fail to see any value marking this clear bug fix as security
related.

On Tue 27-02-24 19:47:26, Greg KH wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> ACPI: custom_method: fix potential use-after-free issue
> 
> In cm_write(), buf is always freed when reaching the end of the
> function.  If the requested count is less than table.length, the
> allocated buffer will be freed but subsequent calls to cm_write() will
> still try to access it.
> 
> Remove the unconditional kfree(buf) at the end of the function and
> set the buf to NULL in the -EINVAL error path to match the rest of
> function.
> 
> The Linux kernel CVE team has assigned CVE-2021-46966 to this issue.
> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Issue introduced in 4.4.195 with commit 4bda2b79a9d0 and fixed in 4.4.269 with commit 1d53ca5d1310
> 	Issue introduced in 4.9.195 with commit 5c12dadcbef8 and fixed in 4.9.269 with commit 8b04d57f30ca
> 	Issue introduced in 4.14.147 with commit 35b88a10535e and fixed in 4.14.233 with commit 90575d1d9311
> 	Issue introduced in 4.19.77 with commit e4467fb6ef54 and fixed in 4.19.191 with commit a5b26a2e362f
> 	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.4.118 with commit 72814a94c38a
> 	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.10.36 with commit 62dc2440ebb5
> 	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.11.20 with commit f16737caf41f
> 	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.12.3 with commit b7a5baaae212
> 	Issue introduced in 5.4 with commit 03d1571d9513 and fixed in 5.13 with commit e483bb9a991b
> 
> Please see https://www.kernel.org or a full list of currently supported
> kernel versions by the kernel community.
> 
> Unaffected versions might change over time as fixes are backported to
> older supported kernel versions.  The official CVE entry at
> 	https://cve.org/CVERecord/?id=CVE-2021-46966
> will be updated if fixes are backported, please check that for the most
> up to date information about this issue.
> 
> 
> Affected files
> ==============
> 
> The file(s) affected by this issue are:
> 	drivers/acpi/custom_method.c
> 
> 
> Mitigation
> ==========
> 
> The Linux kernel CVE team recommends that you update to the latest
> stable kernel version for this, and many other bugfixes.  Individual
> changes are never tested alone, but rather are part of a larger kernel
> release.  Cherry-picking individual commits is not recommended or
> supported by the Linux kernel community at all.  If however, updating to
> the latest release is impossible, the individual changes to resolve this
> issue can be found at these commits:
> 	https://git.kernel.org/stable/c/1d53ca5d131074c925ce38361fb0376d3bf7e394
> 	https://git.kernel.org/stable/c/8b04d57f30caf76649d0567551589af9a66ca9be
> 	https://git.kernel.org/stable/c/90575d1d9311b753cf1718f4ce9061ddda7dfd23
> 	https://git.kernel.org/stable/c/a5b26a2e362f572d87e9fd35435680e557052a17
> 	https://git.kernel.org/stable/c/72814a94c38a33239793f7622cec6ace1e540c4b
> 	https://git.kernel.org/stable/c/62dc2440ebb552aa0d7f635e1697e077d9d21203
> 	https://git.kernel.org/stable/c/f16737caf41fc06cfe6e49048becb09657074d4b
> 	https://git.kernel.org/stable/c/b7a5baaae212a686ceb812c32fceed79c03c0234
> 	https://git.kernel.org/stable/c/e483bb9a991bdae29a0caa4b3a6d002c968f94aa

-- 
Michal Hocko
SUSE Labs