CVE-2024-45025

In the Linux kernel, the following vulnerability has been resolved: fix bitmap corruption on close_range() with CLOSE_RANGE_UNSHARE copy_fd_bitmaps(new, old, count) is expected to copy the first count/BITS_PER_LONG bits from old->full_fds_bits[] and fill the rest with zeroes. What it does is copying enough words (BITS_TO_LONGS(count/BITS_PER_LONG)), then memsets the rest. That works fine, *if* all bits past the cutoff point are clear. Otherwise we are risking garbage from the last word we'd copied. For most of the callers that is true - expand_fdtable() has count equal to old->max_fds, so there's no open descriptors past count, let alone fully occupied words in ->open_fds[], which is what bits in ->full_fds_bits[] correspond to. The other caller (dup_fd()) passes sane_fdtable_size(old_fdt, max_fds), which is the smallest multiple of BITS_PER_LONG that covers all opened descriptors below max_fds. In the common case (copying on fork()) max_fds is ~0U, so all opened descriptors will be below it and we are fine, by the same reasons why the call in expand_fdtable() is safe. Unfortunately, there is a case where max_fds is less than that and where we might, indeed, end up with junk in ->full_fds_bits[] - close_range(from, to, CLOSE_RANGE_UNSHARE) with * descriptor table being currently shared * 'to' being above the current capacity of descriptor table * 'from' being just under some chunk of opened descriptors. In that case we end up with observably wrong behaviour - e.g. spawn a child with CLONE_FILES, get all descriptors in range 0..127 open, then close_range(64, ~0U, CLOSE_RANGE_UNSHARE) and watch dup(0) ending up with descriptor #128, despite #64 being observably not open. The minimally invasive fix would be to deal with that in dup_fd(). If this proves to add measurable overhead, we can go that way, but let's try to fix copy_fd_bitmaps() first. * new helper: bitmap_copy_and_expand(to, from, bits_to_copy, size). * make copy_fd_bitmaps() take the bitmap size in words, rather than bits; it's 'count' argument is always a multiple of BITS_PER_LONG, so we are not losing any information, and that way we can use the same helper for all three bitmaps - compiler will see that count is a multiple of BITS_PER_LONG for the large ones, so it'll generate plain memcpy()+memset(). Reproducer added to tools/testing/selftests/core/close_range_test.c

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References

• MITRE
• NVD
• Launchpad
• Debian

Other references

• https://www.cve.org/CVERecord?id=CVE-2024-45025
• https://git.kernel.org/linus/9a2fa1472083580b6c66bdaf291f591e1170123a (6.11-rc4)
• https://git.kernel.org/stable/c/ee501f827f3db02d4e599afbbc1a7f8b792d05d7
• https://git.kernel.org/stable/c/e807487a1d5fd5d941f26578ae826ca815dbfcd6
• https://git.kernel.org/stable/c/fe5bf14881701119aeeda7cf685f3c226c7380df
• https://git.kernel.org/stable/c/5053581fe5dfb09b58c65dd8462bf5dea71f41ff
• https://git.kernel.org/stable/c/8cad3b2b3ab81ca55f37405ffd1315bcc2948058
• https://git.kernel.org/stable/c/dd72ae8b0fce9c0bbe9582b9b50820f0407f8d8a
• https://git.kernel.org/stable/c/c69d18f0ac7060de724511537810f10f29a27958
• https://git.kernel.org/stable/c/9a2fa1472083580b6c66bdaf291f591e1170123a