CVE-2022-50778

published 2025-12-24

CVE-2022-50778: In the Linux kernel, the following vulnerability has been resolved: fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL With CONFIG_FORTIFY=y and…

Priority P4 26 low 3.3

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

fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL

With CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y enabled, we observe
a runtime panic while running Android's Compatibility Test Suite's (CTS)
android.hardware.input.cts.tests. This is stemming from a strlen()
call in hidinput_allocate().

__compiletime_strlen() is implemented in terms of __builtin_object_size(),
then does an array access to check for NUL-termination. A quirk of
__builtin_object_size() is that for strings whose values are runtime
dependent, __builtin_object_size(str, 1 or 0) returns the maximum size
of possible values when those sizes are determinable at compile time.
Example:

static const char *v = "FOO BAR";
static const char *y = "FOO BA";
unsigned long x (int z) {
// Returns 8, which is:
// max(__builtin_object_size(v, 1), __builtin_object_size(y, 1))
return __builtin_object_size(z ? v : y, 1);
}

So when FORTIFY_SOURCE is enabled, the current implementation of
__compiletime_strlen() will try to access beyond the end of y at runtime
using the size of v. Mixed with UBSAN_LOCAL_BOUNDS we get a fault.

hidinput_allocate() has a local C string whose value is control flow
dependent on a switch statement, so __builtin_object_size(str, 1)
evaluates to the maximum string length, making all other cases fault on
the last character check. hidinput_allocate() could be cleaned up to
avoid runtime calls to strlen() since the local variable can only have
literal values, so there's no benefit to trying to fortify the strlen
call site there.

Perform a __builtin_constant_p() check against index 0 earlier in the
macro to filter out the control-flow-dependant case. Add a KUnit test
for checking the expected behavioral characteristics of FORTIFY_SOURCE
internals.

Affected

10 ranges

Vendor	Product	Version range	Fixed in
debian	linux	< linux 6.0.3-1 (bookworm)	linux 6.0.3-1 (bookworm)
linux	linux	—	—
linux	linux	>= 3009f891bb9f328945ebd5b71e12df7e2467f3dd < ed42391164e6839a48aaf4c53eefda516835e799	ed42391164e6839a48aaf4c53eefda516835e799
linux	linux	>= 3009f891bb9f328945ebd5b71e12df7e2467f3dd < 5d59ad2bfb35fccfe2ad5e8bb8801f6224d3f7d4	5d59ad2bfb35fccfe2ad5e8bb8801f6224d3f7d4
linux	linux	>= 3009f891bb9f328945ebd5b71e12df7e2467f3dd < d07c0acb4f41cc42a0d97530946965b3e4fa68c1	d07c0acb4f41cc42a0d97530946965b3e4fa68c1
linux	linux_kernel	>= 0 < 6.0.3-1	6.0.3-1
linux	linux_kernel	>= 0 < 6.0.3-1	6.0.3-1
linux	linux_kernel	>= 0 < 6.0.3-1	6.0.3-1
linux	linux_kernel	>= 5.16.0 < 5.19.17	5.19.17
linux	linux_kernel	>= 5.20.0 < 6.0.3	6.0.3

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CWE-125