mm/mincore.c: make mincore() more conservative

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;
otherwise we'd be including shared non-exclusive mappings, which

 - is the sidechannel

 - is not the usecase for mincore(), as that's primarily used for data,
   not (shared) text

[[email protected]: v2]
  Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: restructure can_do_mincore() conditions]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Kosina <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Josh Snyder <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Originally-by: Linus Torvalds <[email protected]>
Originally-by: Dominique Martinet <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Kevin Easton <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Cyril Hrubis <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Daniel Gruss <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/mincore.c

@@ -169,6 +169,22 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	return 0;
 }
 
+static inline bool can_do_mincore(struct vm_area_struct *vma)
+{
+	if (vma_is_anonymous(vma))
+		return true;
+	if (!vma->vm_file)
+		return false;
+	/*
+	 * Reveal pagecache information only for non-anonymous mappings that
+	 * correspond to the files the calling process could (if tried) open
+	 * for writing; otherwise we'd be including shared non-exclusive
+	 * mappings, which opens a side channel.
+	 */
+	return inode_owner_or_capable(file_inode(vma->vm_file)) ||
+		inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0;
+}
+
 /*
  * Do a chunk of "sys_mincore()". We've already checked
  * all the arguments, we hold the mmap semaphore: we should
@@ -189,8 +205,13 @@ 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 = DIV_ROUND_UP(end - addr, PAGE_SIZE);
+		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;