Messages in this thread | | | Date | Thu, 7 Mar 2019 01:01:10 +0100 (CET) | From | Jiri Kosina <> | Subject | Re: [PATCH 1/3] mm/mincore: make mincore() more conservative |
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On Wed, 6 Mar 2019, Andrew Morton wrote:
> > The semantics of what mincore() considers to be resident is not completely > > clear, but Linux has always (since 2.3.52, which is when mincore() was > > initially done) treated it as "page is available in page cache". > > > > That's potentially a problem, as that [in]directly exposes meta-information > > about pagecache / memory mapping state even about memory not strictly belonging > > to the process executing the syscall, opening possibilities for sidechannel > > attacks. > > > > Change the semantics of mincore() so that it only reveals pagecache information > > for non-anonymous mappings that belog to files that the calling process could > > (if it tried to) successfully open for writing. > > "for writing" comes as a bit of a surprise. Why not for reading?
I guess this is a rhetorical question from you :) but fair enough, good point, I'll explain this a bit more in the changelog and in the code comments.
> > @@ -189,8 +197,13 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v > > vma = find_vma(current->mm, addr); > > if (!vma || addr < vma->vm_start) > > return -ENOMEM; > > - mincore_walk.mm = vma->vm_mm; > > end = min(vma->vm_end, addr + (pages << PAGE_SHIFT)); > > + if (!can_do_mincore(vma)) { > > + unsigned long pages = (end - addr) >> PAGE_SHIFT; > > I'm not sure this is correct in all cases. If > > addr = 4095 > vma->vm_end = 4096 > pages = 1000 > > then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it > should have been 1.
Good catch! It should rather be something like
unsigned long pages = (end >> PAGE_SHIFT) - (addr >> PAGE_SHIFT);
I'll fix that up and resend tomorrow.
Thanks,
-- Jiri Kosina SUSE Labs
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