CVE-2023-52504

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

x86/alternatives: Disable KASAN in apply_alternatives()

Fei has reported that KASAN triggers during apply_alternatives() on a 5-level paging machine:

BUG: KASAN: out-of-bounds in rcu_is_watching()
Read of size 4 at addr ff110003ee6419a0 by task swapper/0/0
...
__asan_load4()
rcu_is_watching()
trace_hardirqs_on()
text_poke_early()
apply_alternatives()
...

On machines with 5-level paging, cpu_feature_enabled(X86_FEATURE_LA57) gets patched. It includes KASAN code, where KASAN_SHADOW_START depends on __VIRTUAL_MASK_SHIFT, which is defined with cpu_feature_enabled().

KASAN gets confused when apply_alternatives() patches the KASAN_SHADOW_START users. A test patch that makes KASAN_SHADOW_START static, by replacing __VIRTUAL_MASK_SHIFT with 56, works around the issue.

Fix it for real by disabling KASAN while the kernel is patching alternatives.

[ mingo: updated the changelog ]

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/3719d3c36aa853d5a2401af9f8d6b116c91ad5ae
• https://git.kernel.org/stable/c/3770c38cd6a60494da29ac2da73ff8156440a2d1
• https://git.kernel.org/stable/c/5b784489c8158518bf7a466bb3cc045b0fb66b4b
• https://git.kernel.org/stable/c/6788b10620ca6e98575d1e06e72a8974aad7657e
• https://git.kernel.org/stable/c/cd287cc208dfe6bd6da98e7f88e723209242c9b4
• https://git.kernel.org/stable/c/d35652a5fc9944784f6f50a5c979518ff8dacf61
• https://git.kernel.org/stable/c/ecba5afe86f30605eb9dfb7f265a8de0218d4cfc
• https://git.kernel.org/stable/c/3719d3c36aa853d5a2401af9f8d6b116c91ad5ae
• https://git.kernel.org/stable/c/3770c38cd6a60494da29ac2da73ff8156440a2d1
• https://git.kernel.org/stable/c/5b784489c8158518bf7a466bb3cc045b0fb66b4b
• https://git.kernel.org/stable/c/6788b10620ca6e98575d1e06e72a8974aad7657e
• https://git.kernel.org/stable/c/cd287cc208dfe6bd6da98e7f88e723209242c9b4
• https://git.kernel.org/stable/c/d35652a5fc9944784f6f50a5c979518ff8dacf61
• https://git.kernel.org/stable/c/ecba5afe86f30605eb9dfb7f265a8de0218d4cfc