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    SubjectRe: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid pointers
    On (03/15/18 18:35), Linus Torvalds wrote:
    > On Thu, Mar 15, 2018 at 6:18 PM, Sergey Senozhatsky
    > <sergey.senozhatsky.work@gmail.com> wrote:
    > >
    > > Hm, may be sizeof(ptr) still won't suffice. It would be great if we
    > > could always look at spec.field_width, which can be up to 2 * sizeof(void *),
    > > and then just probe_kernel_read(spec.field_width). E.g., %b/%bl prints out a
    > > bitmap, accessing max_t(int, spec.field_width, 0) bits, which is good. But,
    > > for instance, %U (uuid printout) doesn't look at spec.field_width, and reads
    > > in 16 bytes from the given memory address. Then we have ipv4/ipv6, mac, etc.
    > > So I think that checking just 1 byte or sizeof(ptr) is not really enough if
    > > we want to fix vsprintf. What do you think?
    >
    > Honestly, I think it would be better to move the whole logic to the
    > functions that actually do the printout.
    >
    > Then you can do it right, and you don't need to have the strchr() either.
    >
    > There really isn't any commonality between the different versions.
    > field_width is meaningless, since it's about the size of the _printed_
    > field, not the size in memory.

    Agreed!

    > Would it be a few more lines? Yes. But it would also clarify the code
    > and get all the cases right. Look at hex_string() for example, and
    > imagine fetching a byte at a time and just getting the corner cases
    > automatically right.

    So, basically, what I tried to say - any byte past the first sizeof(ptr)
    bytes or past the first byte that we check_access() can cause problems,
    which this patch is trying to address. As an example, FORMAT_TYPE_STR
    case

    printk("%.*s\n", p->buf)
    vsnprintf()
    string()

    Where ->buf is a _nearly always_ properly nul terminated char buf[128]
    array in struct foo. So moving that check_access() to every function that
    does printout sounds good to me, as well as checking every byte we access
    [assuming that we want to cure vsprintf], not just the first one or the
    first sizeof(ptr) bytes.

    -ss

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