🛡️ CVE-2024-50263 — kernel

fork: only invoke khugepaged, ksm hooks if no error

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

fork: only invoke khugepaged, ksm hooks if no error

There is no reason to invoke these hooks early against an mm that is in an

incomplete state.

The change in commit d24062914837 ("fork: use __mt_dup() to duplicate

maple tree in dup_mmap()") makes this more pertinent as we may be in a

state where entries in the maple tree are not yet consistent.

Their placement early in dup_mmap() only appears to have been meaningful

for early error checking, and since functionally it'd require a very small

allocation to fail (in practice 'too small to fail') that'd only occur in

the most dire circumstances, meaning the fork would fail or be OOM'd in

any case.

Since both khugepaged and KSM tracking are there to provide optimisations

to memory performance rather than critical functionality, it doesn't

really matter all that much if, under such dire memory pressure, we fail

to register an mm with these.

As a result, we follow the example of commit d2081b2bf819 ("mm:

khugepaged: make khugepaged_enter() void function") and make ksm_fork() a

void function also.

We only expose the mm to these functions once we are done with them and

only if no error occurred in the fork operation.