[patch 24/88] mm: do_xip_mapping_read: fix length calculation

[Greg KH]

2.6.28-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>

upstream commit: 58984ce21d315b70df1a43644df7416ea7c9bfd8

The calculation of the value nr in do_xip_mapping_read is incorrect.  If
the copy required more than one iteration in the do while loop the copies
variable will be non-zero.  The maximum length that may be passed to the
call to copy_to_user(buf+copied, xip_mem+offset, nr) is len-copied but the
check only compares against (nr > len).

This bug is the cause for the heap corruption Carsten has been chasing
for so long:

 *** glibc detected *** /bin/bash: free(): invalid next size (normal): 0x00000000800e39f0 ***
 ======= Backtrace: =========
/lib64/libc.so.6[0x200000b9b44]
/lib64/libc.so.6(cfree+0x8e)[0x200000bdade]
/bin/bash(free_buffered_stream+0x32)[0x80050e4e]
/bin/bash(close_buffered_stream+0x1c)[0x80050ea4]
/bin/bash(unset_bash_input+0x2a)[0x8001c366]
/bin/bash(make_child+0x1d4)[0x8004115c]
/bin/bash[0x8002fc3c]
/bin/bash(execute_command_internal+0x656)[0x8003048e]
/bin/bash(execute_command+0x5e)[0x80031e1e]
/bin/bash(execute_command_internal+0x79a)[0x800305d2]
/bin/bash(execute_command+0x5e)[0x80031e1e]
/bin/bash(reader_loop+0x270)[0x8001efe0]
/bin/bash(main+0x1328)[0x8001e960]
/lib64/libc.so.6(__libc_start_main+0x100)[0x200000592a8]
/bin/bash(clearerr+0x5e)[0x8001c092]

With this bug fix the commit 0e4a9b59282914fe057ab17027f55123964bc2e2
"ext2/xip: refuse to change xip flag during remount with busy inodes" can
be removed again.

Cc: Carsten Otte <cotte@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxx>
Cc: Jared Hulbert <jaredeh@xxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 mm/filemap_xip.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -89,8 +89,8 @@ do_xip_mapping_read(struct address_space
 			}
 		}
 		nr = nr - offset;
-		if (nr > len)
-			nr = len;
+		if (nr > len - copied)
+			nr = len - copied;
 
 		error = mapping->a_ops->get_xip_mem(mapping, index, 0,
 							&xip_mem, &xip_pfn);


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