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author | Hugh Dickins <hugh@veritas.com> | 2009-01-06 14:39:32 -0800 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2009-01-06 15:59:03 -0800 |
commit | 878b63ac889df706d01048f2c110e322ad2f996d (patch) | |
tree | e0bd69b890fda939fc190675d977dd0fd75e0427 /mm | |
parent | 2da02997e08d3efe8174c7a47696e6f7cbe69ba9 (diff) | |
download | linux-878b63ac889df706d01048f2c110e322ad2f996d.tar.gz linux-878b63ac889df706d01048f2c110e322ad2f996d.tar.bz2 linux-878b63ac889df706d01048f2c110e322ad2f996d.zip |
mm: gup persist for write permission
do_wp_page()'s VM_FAULT_WRITE return value tells __get_user_pages() that
COW has been done if necessary, though it may be leaving the pte without
write permission - for the odd case of forced writing to a readonly vma
for ptrace. At present GUP then retries the follow_page() without asking
for write permission, to escape an endless loop when forced.
But an application may be relying on GUP to guarantee a writable page
which won't be COWed again when written from userspace, whereas a race
here might leave a readonly pte in place? Change the VM_FAULT_WRITE
handling to ask follow_page() for write permission again, except in that
odd case of forced writing to a readonly vma.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r-- | mm/memory.c | 10 |
1 files changed, 8 insertions, 2 deletions
diff --git a/mm/memory.c b/mm/memory.c index 122d965e820f..f594bb65a9f1 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1264,9 +1264,15 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, * do_wp_page has broken COW when necessary, * even if maybe_mkwrite decided not to set * pte_write. We can thus safely do subsequent - * page lookups as if they were reads. + * page lookups as if they were reads. But only + * do so when looping for pte_write is futile: + * in some cases userspace may also be wanting + * to write to the gotten user page, which a + * read fault here might prevent (a readonly + * page might get reCOWed by userspace write). */ - if (ret & VM_FAULT_WRITE) + if ((ret & VM_FAULT_WRITE) && + !(vma->vm_flags & VM_WRITE)) foll_flags &= ~FOLL_WRITE; cond_resched(); |