CVE-2024-43882

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

exec: Fix ToCToU between perm check and set-uid/gid usage

When opening a file for exec via do_filp_open(), permission checking is

done against the file's metadata at that moment, and on success, a file

pointer is passed back. Much later in the execve() code path, the file

metadata (specifically mode, uid, and gid) is used to determine if/how

to set the uid and gid. However, those values may have changed since the

permissions check, meaning the execution may gain unintended privileges.

For example, if a file could change permissions from executable and not

set-id:

---------x 1 root root 16048 Aug 7 13:16 target

to set-id and non-executable:

---S------ 1 root root 16048 Aug 7 13:16 target

it is possible to gain root privileges when execution should have been

disallowed.

While this race condition is rare in real-world scenarios, it has been

observed (and proven exploitable) when package managers are updating

the setuid bits of installed programs. Such files start with being

world-executable but then are adjusted to be group-exec with a set-uid

bit. For example, "chmod o-x,u+s target" makes "target" executable only

by uid "root" and gid "cdrom", while also becoming setuid-root:

-rwxr-xr-x 1 root cdrom 16048 Aug 7 13:16 target

becomes:

-rwsr-xr-- 1 root cdrom 16048 Aug 7 13:16 target

But racing the chmod means users without group "cdrom" membership can

get the permission to execute "target" just before the chmod, and when

the chmod finishes, the exec reaches brpm_fill_uid(), and performs the

setuid to root, violating the expressed authorization of "only cdrom

group members can setuid to root".

Re-check that we still have execute permissions in case the metadata

has changed. It would be better to keep a copy from the perm-check time,

but until we can do that refactoring, the least-bad option is to do a

full inode_permission() call (under inode lock). It is understood that

this is safe against dead-locks, but hardly optimal.