Messages in this thread | | | Date | Fri, 16 Mar 2018 14:53:46 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH v3] vsprintf: Prevent crash when dereferencing invalid pointers |
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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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