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    SubjectRe: [PATCH] x86/mpx: fix recursive munmap() corruption
    Date
    Thomas Gleixner <tglx@linutronix.de> writes:
    > On Mon, 1 Apr 2019, Dave Hansen wrote:
    >> diff -puN mm/mmap.c~mpx-rss-pass-no-vma mm/mmap.c
    >> --- a/mm/mmap.c~mpx-rss-pass-no-vma 2019-04-01 06:56:53.409411123 -0700
    >> +++ b/mm/mmap.c 2019-04-01 06:56:53.423411123 -0700
    >> @@ -2731,9 +2731,17 @@ int __do_munmap(struct mm_struct *mm, un
    >> return -EINVAL;
    >>
    >> len = PAGE_ALIGN(len);
    >> + end = start + len;
    >> if (len == 0)
    >> return -EINVAL;
    >>
    >> + /*
    >> + * arch_unmap() might do unmaps itself. It must be called
    >> + * and finish any rbtree manipulation before this code
    >> + * runs and also starts to manipulate the rbtree.
    >> + */
    >> + arch_unmap(mm, start, end);
    >
    > ...
    >
    >> -static inline void arch_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
    >> - unsigned long start, unsigned long end)
    >> +static inline void arch_unmap(struct mm_struct *mm, unsigned long start,
    >> + unsigned long end)
    >
    > While you fixed up the asm-generic thing, this breaks arch/um and
    > arch/unicorn32. For those the fixup is trivial by removing the vma
    > argument.
    >
    > But itt also breaks powerpc and there I'm not sure whether moving
    > arch_unmap() to the beginning of __do_munmap() is safe. Micheal???

    I don't know for sure but I think it should be fine. That code is just
    there to handle CRIU unmapping/remapping the VDSO. So that either needs
    to happen while the process is stopped or it needs to handle races
    anyway, so I don't see how the placement within the unmap path should
    matter.

    > Aside of that the powerpc variant looks suspicious:
    >
    > static inline void arch_unmap(struct mm_struct *mm,
    > unsigned long start, unsigned long end)
    > {
    > if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
    > mm->context.vdso_base = 0;
    > }
    >
    > Shouldn't that be:
    >
    > if (start >= mm->context.vdso_base && mm->context.vdso_base < end)
    >
    > Hmm?

    Yeah looks pretty suspicious. I'll follow-up with Laurent who wrote it.
    Thanks for spotting it!

    cheers

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