[RFC PATCH v1 0/2] Fix offset when fault occurs in strncpy_from_kernel_nofault()

[Francis Laniel <flaniel@linux.microsoft.com>]

Hi.


First of all, I hope you are fine and the same for your relatives.

This contribution fixes a bug where the byte before the destination address can
be reset when a page fault occurs in strncpy_from_kernel_nofault() while copying
the first byte from the source address.

This bug leaded to kernel panic if a pointer containing the modified address is
dereferenced as the pointer does not contain a correct addresss.

To fix this bug, we simply reset the current destination byte in a case of a
page fault.
The proposed fix was tested and validated inside a VM:
root@vm-amd64:~# ./share/linux/tools/testing/selftests/bpf/test_progs --name varlen
...
#222     varlen:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
Without the patch, the test will fail:
root@vm-amd64:~# ./share/linux/tools/testing/selftests/bpf/test_progs --name varlen
...
#222     varlen:FAIL
Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED

If you see any way to improve this contribution, feel free to share.

Alban Crequy (2):
  maccess: fix writing offset in case of fault in
    strncpy_from_kernel_nofault()
  selftests: bpf: add a test when bpf_probe_read_kernel_str() returns
    EFAULT

 mm/maccess.c                                    | 2 +-
 tools/testing/selftests/bpf/prog_tests/varlen.c | 7 +++++++
 tools/testing/selftests/bpf/progs/test_varlen.c | 5 +++++
 3 files changed, 13 insertions(+), 1 deletion(-)


Best regards and thank you in advance.
--
2.25.1