CVE-2024-35801

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

x86/fpu: Keep xfd_state in sync with MSR_IA32_XFD

Commit 672365477ae8 (x86/fpu: Update XFD state where required) and commit 8bf26758ca96 (x86/fpu: Add XFD state to fpstate) introduced a per CPU variable xfd_state to keep the MSR_IA32_XFD value cached, in order to avoid unnecessary writes to the MSR.

On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which wipes out any stale state. But the per CPU cached xfd value is not reset, which brings them out of sync.

As a consequence a subsequent xfd_update_state() might fail to update the MSR which in turn can result in XRSTOR raising a #NM in kernel space, which crashes the kernel.

To fix this, introduce xfd_set_state() to write xfd_state together with MSR_IA32_XFD, and use it in all places that set MSR_IA32_XFD.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/10e4b5166df9ff7a2d5316138ca668b42d004422
• https://git.kernel.org/stable/c/1acbca933313aa866e39996904c9aca4d435c4cd
• https://git.kernel.org/stable/c/21c7c00dae55cb0e3810d5f9506b58f68475d41d
• https://git.kernel.org/stable/c/92b0f04e937665bde5768f3fcc622dcce44413d8
• https://git.kernel.org/stable/c/b61e3b7055ac6edee4be071c52f48c26472d2624