CVE-2024-50182

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

secretmem: disable memfd_secret() if arch cannot set direct map

Return -ENOSYS from memfd_secret() syscall if !can_set_direct_map(). This

is the case for example on some arm64 configurations, where marking 4k

PTEs in the direct map not present can only be done if the direct map is

set up at 4k granularity in the first place (as ARM's break-before-make

semantics do not easily allow breaking apart large/gigantic pages).

More precisely, on arm64 systems with !can_set_direct_map(),

set_direct_map_invalid_noflush() is a no-op, however it returns success

(0) instead of an error. This means that memfd_secret will seemingly

"work" (e.g. syscall succeeds, you can mmap the fd and fault in pages),

but it does not actually achieve its goal of removing its memory from the

direct map.

Note that with this patch, memfd_secret() will start erroring on systems

where can_set_direct_map() returns false (arm64 with

CONFIG_RODATA_FULL_DEFAULT_ENABLED=n, CONFIG_DEBUG_PAGEALLOC=n and

CONFIG_KFENCE=n), but that still seems better than the current silent

failure. Since CONFIG_RODATA_FULL_DEFAULT_ENABLED defaults to 'y', most

arm64 systems actually have a working memfd_secret() and aren't be

affected.

From going through the iterations of the original memfd_secret patch

series, it seems that disabling the syscall in these scenarios was the

intended behavior [1] (preferred over having

set_direct_map_invalid_noflush return an error as that would result in

SIGBUSes at page-fault time), however the check for it got dropped between

v16 [2] and v17 [3], when secretmem moved away from CMA allocations.

[1]: https://lore.kernel.org/lkml/[email protected]/

[2]: https://lore.kernel.org/lkml/[email protected]/#t

[3]: https://lore.kernel.org/lkml/[email protected]/