Messages in this thread | | | Date | Sat, 19 Sep 2020 01:17:14 +0100 | From | Al Viro <> | Subject | Re: WARNING in ex_handler_uaccess |
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On Fri, Sep 18, 2020 at 05:07:43PM -0700, Andy Lutomirski wrote: > On Fri, Sep 18, 2020 at 4:55 PM Al Viro <viro@zeniv.linux.org.uk> wrote: > > > > On Fri, Sep 18, 2020 at 04:31:33PM -0700, Andy Lutomirski wrote: > > > > > check_zeroed_user() looks buggy. It does: > > > > > > if (!user_access_begin(from, size)) > > > return -EFAULT; > > > > > > unsafe_get_user(val, (unsigned long __user *) from, err_fault); > > > > > > This is wrong if size < sizeof(unsigned long) -- you read outside the > > > area you verified using user_access_begin(). > > > > Read the code immediately prior to that. from will be word-aligned, > > and size will be extended accordingly. If the area acceptable for > > user_access_begin() ends *NOT* on a word boundary, you have a problem > > and I would strongly recommend to seek professional help. > > > > All reads in that thing are word-aligned and word-sized. So I very > > much doubt that your analysis is correct. > > Maybe -ETOOTIRED, but I seriously question the math in here. Suppose > from == (unsigned long *)1 and size == 1. Then align is 1, and we do: > > from -= align; > size += align; > > So now from = 0 and size = 2. Now we do user_access_begin(0, 2) and > then immediately read 4 or 8 bytes. No good.
Could you explain what kind of insane hardware manages to do #PF-related checks (including SMAP, whatever) with *sub*WORD* granularity?
If it's OK with 16bit read from word-aligned address, but barfs on 64bit one... I want to know what the hell had its authors been smoking.
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