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Date: Wed, 6 Mar 2019 15:13:51 -0800 From: Andrew Morton <akpm@...ux-foundation.org> To: Vlastimil Babka <vbabka@...e.cz> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org>, Jann Horn <jannh@...gle.com>, Jiri Kosina <jkosina@...e.cz>, Dominique Martinet <asmadeus@...ewreck.org>, Andy Lutomirski <luto@...capital.net>, Dave Chinner <david@...morbit.com>, Kevin Easton <kevin@...rana.org>, Matthew Wilcox <willy@...radead.org>, Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>, "Kirill A . Shutemov" <kirill@...temov.name>, Daniel Gruss <daniel@...ss.cc>, Jiri Kosina <jikos@...nel.org> Subject: Re: [PATCH 1/3] mm/mincore: make mincore() more conservative On Wed, 30 Jan 2019 13:44:18 +0100 Vlastimil Babka <vbabka@...e.cz> wrote: > From: Jiri Kosina <jkosina@...e.cz> > > 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? Could we please explain the reasoning in the changelog and in the (presently absent) comments which describe can_do_mincore()? > @@ -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. Please check? A mincore test suite in tools/testing/selftests would be useful, methinks. To exercise such corner cases, check for future breakage, etc. > + memset(vec, 1, pages); > + return pages; > + } > + mincore_walk.mm = vma->vm_mm; > err = walk_page_range(addr, end, &mincore_walk); > if (err < 0) > return err;
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