CVE-2026-46063

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

x86/shstk: Prevent deadlock during shstk sigreturn

During sigreturn the shadow stack signal frame is popped. The kernel does

this by reading the shadow stack using normal read accesses. When it can't

assume the memory is shadow stack, it takes extra steps to makes sure it is

reading actual shadow stack memory and not other normal readable memory. It

does this by holding the mmap read lock while doing the access and checking

the flags of the VMA.

Unfortunately that is not safe. If the read of the shadow stack sigframe

hits a page fault, the fault handler will try to recursively grab another

mmap read lock. This normally works ok, but if a writer on another CPU is

also waiting, the second read lock could fail and cause a deadlock.

Fix this by not holding mmap lock during the read access to userspace.

Instead use mmap_lock_speculate_...() to watch for changes between dropping

mmap lock and the userspace access. Retry if anything grabbed an mmap write

lock in between and could have changed the VMA.

These mmap_lock_speculate_...() helpers use mm::mm_lock_seq, which is only

available when PER_VMA_LOCK is configured. So make X86_USER_SHADOW_STACK

depend on it. On x86, PER_VMA_LOCK is a default configuration for SMP

kernels. So drop support for the other configs under the assumption that

the !SMP shadow stack user base does not exist.

Currently there is a check that skips the lookup work when the SSP can be

assumed to be on a shadow stack. While reorganizing the function, remove

the optimization to make the tricky code flows more common, such that

issues like this cannot escape detection for so long.