CVE-2024-46847 — mm: vmalloc: ensure vmap_block is initialised before adding to queue

Affected: Linux

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

mm: vmalloc: ensure vmap_block is initialised before adding to queue

Commit 8c61291fd850 ("mm: fix incorrect vbq reference in purge_fragmented_block") extended the 'vmap_block' structure to contain a 'cpu' field which is set at allocation time to the id of the initialising CPU.

When a new 'vmap_block' is being instantiated by new_vmap_block(), the partially initialised structure is added to the local 'vmap_block_queue' xarray before the 'cpu' field has been initialised. If another CPU is concurrently walking the xarray (e.g. via vm_unmap_aliases()), then it may perform an out-of-bounds access to the remote queue thanks to an uninitialised index.

This has been observed as UBSAN errors in Android:

Move the initialisation of 'vb->cpu' in new_vmap_block() ahead of the addition to the xarray.

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CVE-2024-46847 — mm: vmalloc: ensure vmap_block is initialised before adding to queue