mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge

Summary

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

mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge

Patch series "mm/vma: fix anon_vma UAF on mremap() faulted, unfaulted merge", v2.

Commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges") introduced the ability to merge previously unavailable VMA merge scenarios.

However, it is handling merges incorrectly when it comes to mremap() of a faulted VMA adjacent to an unfaulted VMA. The issues arise in three cases:

1. Previous VMA unfaulted:

copied -----| v |-----------|.............| | unfaulted |(faulted VMA)| |-----------|.............| prev

2. Next VMA unfaulted:

copied -----| v |.............|-----------| |(faulted VMA)| unfaulted | |.............|-----------| next

3. Both adjacent VMAs unfaulted:

copied -----| v |-----------|.............|-----------| | unfaulted |(faulted VMA)| unfaulted | |-----------|.............|-----------| prev next

This series fixes each of these cases, and introduces self tests to assert that the issues are corrected.

I also test a further case which was already handled, to assert that my changes continues to correctly handle it:

4. prev unfaulted, next faulted:

copied -----| v |-----------|.............|-----------| | unfaulted |(faulted VMA)| faulted | |-----------|.............|-----------| prev next

This bug was discovered via a syzbot report, linked to in the first patch in the series, I confirmed that this series fixes the bug.

I also discovered that we are failing to check that the faulted VMA was not forked when merging a copied VMA in cases 1-3 above, an issue this series also addresses.

I also added self tests to assert that this is resolved (and confirmed that the tests failed prior to this).

I also cleaned up vma_expand() as part of this work, renamed vma_had_uncowed_parents() to vma_is_fork_child() as the previous name was unduly confusing, and simplified the comments around this function.

This patch (of 4):

Commit 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges") introduced the ability to merge previously unavailable VMA merge scenarios.

The key piece of logic introduced was the ability to merge a faulted VMA immediately next to an unfaulted VMA, which relies upon dup_anon_vma() to correctly handle anon_vma state.

In the case of the merge of an existing VMA (that is changing properties of a VMA and then merging if those properties are shared by adjacent VMAs), dup_anon_vma() is invoked correctly.

However in the case of the merge of a new VMA, a corner case peculiar to mremap() was missed.

The issue is that vma_expand() only performs dup_anon_vma() if the target (the VMA that will ultimately become the merged VMA): is not the next VMA, i.e. the one that appears after the range in which the new VMA is to be established.

A key insight here is that in all other cases other than mremap(), a new VMA merge either expands an existing VMA, meaning that the target VMA will be that VMA, or would have anon_vma be NULL.

Specifically:

* __mmap_region() - no anon_vma in place, initial mapping. * do_brk_flags() - expanding an existing VMA. * vma_merge_extend() - expanding an existing VMA. * relocate_vma_down() - no anon_vma in place, initial mapping.

In addition, we are in the unique situation of needing to duplicate anon_vma state from a VMA that is neither the previous or next VMA being merged with.

dup_anon_vma() deals exclusively with the target=unfaulted, src=faulted case. This leaves four possibilities, in each case where the copied VMA is faulted:

1. Previous VMA unfaulted:

copied -----|

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