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[PATCH] fs: fix libfs data leak

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simple_prepare_write leaks uninitialised kernel data.  This happens because
the it leaves an uninitialised "hole" over the part of the page that the
write is expected to go to.  This is fine, but it then marks the page
uptodate, which means a concurrent read can come in and copy the
uninitialised memory into userspace before it written to.

Fix it by simply marking it uptodate in simple_commit_write instead, after
the hole has been filled in.  This could theoretically break an fs that
uses simple_prepare_write and not simple_commit_write, and that relies on
the incorrect simple_prepare_write behaviour.  Luckily, none of those
exists in the tree.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nick Piggin 2007-02-20 13:58:08 -08:00 committed by Linus Torvalds
parent c066332fb1
commit 955eff5acc
2 changed files with 8 additions and 2 deletions
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5
Documentation/filesystems/vfs.txt
View file

@ -617,6 +617,11 @@ struct address_space_operations {
In this case the prepare_write will be retried one the lock is
regained.
Note: the page _must not_ be marked uptodate in this function
(or anywhere else) unless it actually is uptodate right now. As
soon as a page is marked uptodate, it is possible for a concurrent
read(2) to copy it to userspace.
commit_write: If prepare_write succeeds, new data will be copied
into the page and then commit_write will be called. It will
typically update the size of the file (if appropriate) and

5
fs/libfs.c
View file

@ -335,17 +335,18 @@ int simple_prepare_write(struct file *file, struct page *page,
flush_dcache_page(page);
kunmap_atomic(kaddr, KM_USER0);
}
SetPageUptodate(page);
}
return 0;
}
int simple_commit_write(struct file *file, struct page *page,
unsigned offset, unsigned to)
unsigned from, unsigned to)
{
struct inode *inode = page->mapping->host;
loff_t pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
if (!PageUptodate(page))
SetPageUptodate(page);
/*
* No need to use i_size_read() here, the i_size
* cannot change under us because we hold the i_mutex.