linux-kernel - Re: [PATCH] befs: redundant test on unsigned in befs_get

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Date:	Tue, 20 Oct 2009 23:17:51 +0200
From:	Roel Kluin <roel.kluin@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"Sergey S. Kostyliov" <rathamahata@...4.ru>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] befs: redundant test on unsigned in befs_get_block()?

block is unsigned, check whether it is not too large.

Signed-off-by: Roel Kluin <roel.kluin@...il.com>
---
> As far as the VFS is concerned, `block' is indeed unsigned and may well
> be in the range 2G-4G with a 32-bit sector_t.  Perhaps not possible on
> befs but still legal to the VFS.
> 
> So the test is wrong from that POV.
> 
> However it is possible that befs is defending itself here.  Perhaps code
> internal to befs will explode if passed a "negative" block number.  Due
> to coding errors within the fs implementation.
> 
> So really, we'd need to check all code paths called by
> befs_get_block() and check that they are signednessly clean.

This appears to be already noted by Jesper Juhl in 2004, however it
was never fixed: http://search.luky.org/linux-kernel.2004/msg01392.html

It's getting late here, but what do you think about this:
befs_get_block() calls befs_fblock2brun() and there occurs a 


pos = fblock << BEFS_SB(sb)->block_shift;

and in effect:
if (pos >= data->max_double_indirect_range)
	error out.

So if I'm not mistaken, this should provide protection:

diff --git a/fs/befs/linuxvfs.c b/fs/befs/linuxvfs.c
index 33baf27..eeb4625 100644
--- a/fs/befs/linuxvfs.c
+++ b/fs/befs/linuxvfs.c
@@ -128,9 +128,9 @@ befs_get_block(struct inode *inode, sector_t block,
 	befs_debug(sb, "---> befs_get_block() for inode %lu, block %ld",
 		   inode->i_ino, block);
 
-	if (block < 0) {
-		befs_error(sb, "befs_get_block() was asked for a block "
-			   "number less than zero: block %ld in inode %lu",
+	if (block >= ds->max_double_indirect_range >>
+			BEFS_SB(sb)->block_shift) {
+		befs_error(sb, "befs_get_block() was asked for a too large block: block %ld in inode %lu",
 			   block, inode->i_ino);
 		return -EIO;
 	}
--
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