Commit 955eff5a authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds

[PATCH] fs: fix libfs data leak

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: default avatarNick Piggin <npiggin@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent c066332f
...@@ -617,6 +617,11 @@ struct address_space_operations { ...@@ -617,6 +617,11 @@ struct address_space_operations {
In this case the prepare_write will be retried one the lock is In this case the prepare_write will be retried one the lock is
regained. 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 commit_write: If prepare_write succeeds, new data will be copied
into the page and then commit_write will be called. It will into the page and then commit_write will be called. It will
typically update the size of the file (if appropriate) and typically update the size of the file (if appropriate) and
......
...@@ -335,17 +335,18 @@ int simple_prepare_write(struct file *file, struct page *page, ...@@ -335,17 +335,18 @@ int simple_prepare_write(struct file *file, struct page *page,
flush_dcache_page(page); flush_dcache_page(page);
kunmap_atomic(kaddr, KM_USER0); kunmap_atomic(kaddr, KM_USER0);
} }
SetPageUptodate(page);
} }
return 0; return 0;
} }
int simple_commit_write(struct file *file, struct page *page, int simple_commit_write(struct file *file, struct page *page,
unsigned offset, unsigned to) unsigned from, unsigned to)
{ {
struct inode *inode = page->mapping->host; struct inode *inode = page->mapping->host;
loff_t pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to; 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 * No need to use i_size_read() here, the i_size
* cannot change under us because we hold the i_mutex. * cannot change under us because we hold the i_mutex.
......
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