linux-cve-announce - CVE-2022-49520: arm64: compat: Do not treat syscall number as ESR

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Message-ID: <2025022611-CVE-2022-49520-465c@gregkh>
Date: Wed, 26 Feb 2025 03:13:14 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2022-49520: arm64: compat: Do not treat syscall number as ESR_ELx for a bad syscall

Description
===========

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

arm64: compat: Do not treat syscall number as ESR_ELx for a bad syscall

If a compat process tries to execute an unknown system call above the
__ARM_NR_COMPAT_END number, the kernel sends a SIGILL signal to the
offending process. Information about the error is printed to dmesg in
compat_arm_syscall() -> arm64_notify_die() -> arm64_force_sig_fault() ->
arm64_show_signal().

arm64_show_signal() interprets a non-zero value for
current->thread.fault_code as an exception syndrome and displays the
message associated with the ESR_ELx.EC field (bits 31:26).
current->thread.fault_code is set in compat_arm_syscall() ->
arm64_notify_die() with the bad syscall number instead of a valid ESR_ELx
value. This means that the ESR_ELx.EC field has the value that the user set
for the syscall number and the kernel can end up printing bogus exception
messages*. For example, for the syscall number 0x68000000, which evaluates
to ESR_ELx.EC value of 0x1A (ESR_ELx_EC_FPAC) the kernel prints this error:

[   18.349161] syscall[300]: unhandled exception: ERET/ERETAA/ERETAB, ESR 0x68000000, Oops - bad compat syscall(2) in syscall[10000+50000]
[   18.350639] CPU: 2 PID: 300 Comm: syscall Not tainted 5.18.0-rc1 #79
[   18.351249] Hardware name: Pine64 RockPro64 v2.0 (DT)
[..]

which is misleading, as the bad compat syscall has nothing to do with
pointer authentication.

Stop arm64_show_signal() from printing exception syndrome information by
having compat_arm_syscall() set the ESR_ELx value to 0, as it has no
meaning for an invalid system call number. The example above now becomes:

[   19.935275] syscall[301]: unhandled exception: Oops - bad compat syscall(2) in syscall[10000+50000]
[   19.936124] CPU: 1 PID: 301 Comm: syscall Not tainted 5.18.0-rc1-00005-g7e08006d4102 #80
[   19.936894] Hardware name: Pine64 RockPro64 v2.0 (DT)
[..]

which although shows less information because the syscall number,
wrongfully advertised as the ESR value, is missing, it is better than
showing plainly wrong information. The syscall number can be easily
obtained with strace.

*A 32-bit value above or equal to 0x8000_0000 is interpreted as a negative
integer in compat_arm_syscal() and the condition scno < __ARM_NR_COMPAT_END
evaluates to true; the syscall will exit to userspace in this case with the
ENOSYS error code instead of arm64_notify_die() being called.

The Linux kernel CVE team has assigned CVE-2022-49520 to this issue.

Affected and fixed versions
===========================

	Fixed in 5.4.198 with commit efd183d988b416fcdf6f7c298a17ced4859ca77d
	Fixed in 5.10.121 with commit ad97425d23af3c3b8d4f6a2bb666cb485087c007
	Fixed in 5.15.46 with commit 621916afe8cd4f322eb12759b64a2f938d4e551d
	Fixed in 5.17.14 with commit 095e975f8150ccd7f852eb578c1cdbdd2f517c7a
	Fixed in 5.18.3 with commit 3910ae71cb963fa2b68e684489d4fc3d105afda0
	Fixed in 5.19 with commit 3fed9e551417b84038b15117732ea4505eee386b

Please see https://www.kernel.org for 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-2022-49520
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:
	arch/arm64/kernel/sys_compat.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/efd183d988b416fcdf6f7c298a17ced4859ca77d
	https://git.kernel.org/stable/c/ad97425d23af3c3b8d4f6a2bb666cb485087c007
	https://git.kernel.org/stable/c/621916afe8cd4f322eb12759b64a2f938d4e551d
	https://git.kernel.org/stable/c/095e975f8150ccd7f852eb578c1cdbdd2f517c7a
	https://git.kernel.org/stable/c/3910ae71cb963fa2b68e684489d4fc3d105afda0
	https://git.kernel.org/stable/c/3fed9e551417b84038b15117732ea4505eee386b