On Thu, Sep 08, 2022 at 11:48:09PM +0300, Sergei Antonov wrote:
> Running this test program on ARMv4 a few times (sometimes just once)
> reproduces the bug.
>
> int main()
> {
> unsigned i;
> char paragon[SIZE];
> void* ptr;
>
> memset(paragon, 0xAA, SIZE);
> ptr = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> MAP_ANON | MAP_SHARED, -1, 0);
> if (ptr == MAP_FAILED) return 1;
> printf("ptr = %p\n", ptr);
> for (i=0;i<10000;i++){
> memset(ptr, 0xAA, SIZE);
> if (memcmp(ptr, paragon, SIZE)) {
> printf("Unexpected bytes on iteration %u!!!\n", i);
> break;
> }
> }
> munmap(ptr, SIZE);
> }
>
> In the "ptr" buffer there appear runs of zero bytes which are aligned
> by 16 and their lengths are multiple of 16.
>
> Linux v5.11 does not have the bug, "git bisect" finds the first bad commit:
> f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
>
> Before the commit update_mmu_cache() was called during a call to
> filemap_map_pages() as well as finish_fault(). After the commit
> finish_fault() lacks it.
>
> Bring back update_mmu_cache() to finish_fault() to fix the bug.
> Also call update_mmu_tlb() only when returning VM_FAULT_NOPAGE to more
> closely reproduce the code of alloc_set_pte() function that existed before
> the commit.
>
> On many platforms update_mmu_cache() is nop:
> x86, see arch/x86/include/asm/pgtable
> ARMv6+, see arch/arm/include/asm/tlbflush.h
> So, it seems, few users ran into this bug.
>
> Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
> Signed-off-by: Sergei Antonov <saproj@xxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
> mm/memory.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
<formletter>
This is not the correct way to submit patches for inclusion in the
stable kernel tree. Please read:
https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.
</formletter>
