CVE-2026-23077: 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 -----|                         ---truncated---