CVE-2024-26603

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

x86/fpu: Stop relying on userspace for info to fault in xsave buffer

Before this change, the expected size of the user space buffer was taken from fxsw->xstatesize. fxsw->xstatesize can be changed from user-space, so it is possible construct a sigreturn frame where:

• fxsw->xstatesize is smaller than the size required by valid bits in fx_sw->xfeatures.
• user-space unmaps parts of the sigrame fpu buffer so that not all of the buffer required by xrstor is accessible.

In this case, xrstor tries to restore and accesses the unmapped area which results in a fault. But faultinreadable succeeds because buf + fxsw->xstatesize is within the still mapped area, so it goes back and tries xrstor again. It will spin in this loop forever.

Instead, fault in the maximum size which can be touched by XRSTOR (taken from fpstate->user_size).

[ dhansen: tweak subject / changelog ]