Re: [PATCH] do_xip_mapping_read: fix length calculation

[Minchan Kim]

Nice catch!

I tested this patch on ext2's ramdisk with xip.
It fixed following bug.

*** stack smashing detected ***: <unknown> terminated
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x48)[0xb7fe5138]
/lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x0)[0xb7fe50f0]
[0x80485f8]
[0x0]
======= Memory map: ========
08048000-08049000 r-xp 00000000 01:00 12         /mnt/xip_test
08049000-0804a000 rw-p 00000000 01:00 12         /mnt/xip_test
0804a000-0806b000 rw-p 0804a000 00:00 0          [heap]
b7ef7000-b7ef8000 rw-p b7ef7000 00:00 0
b7ef8000-b8041000 r-xp 00000000 08:03 4219915    /lib/tls/i686/cmov/libc-2.7.so
b8041000-b8042000 r--p 00149000 08:03 4219915    /lib/tls/i686/cmov/libc-2.7.so
b8042000-b8044000 rw-p 0014a000 08:03 4219915    /lib/tls/i686/cmov/libc-2.7.so
b8044000-b8047000 rw-p b8044000 00:00 0
b8047000-b8051000 r-xp 00000000 08:03 4202508    /lib/libgcc_s.so.1
b8051000-b8052000 rw-p 0000a000 08:03 4202508    /lib/libgcc_s.so.1
b8052000-b8055000 rw-p b8052000 00:00 0
b8055000-b8056000 r-xp b8055000 00:00 0          [vdso]
b8056000-b8070000 r-xp 00000000 08:03 4202565    /lib/ld-2.7.so
b8070000-b8072000 rw-p 00019000 08:03 4202565    /lib/ld-2.7.so
bf95c000-bf971000 rw-p bffeb000 00:00 0          [stack]
Aborted


Reviewed-by: Minchan kim <minchan.kim@xxxxxxxxx>
Tested-by: Minchan kim <minchan.kim@xxxxxxxxx>


On Tue, Mar 31, 2009 at 10:32 PM, Martin Schwidefsky
<schwidefsky@xxxxxxxxxx> wrote:
> From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
>
> 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>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> ---
>
> Âmm/filemap_xip.c | Â Â4 ++--
> Â1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff -urpN linux-2.6/mm/filemap_xip.c linux-2.6-patched/mm/filemap_xip.c
> --- linux-2.6/mm/filemap_xip.c Â2009-03-24 00:12:14.000000000 +0100
> +++ linux-2.6-patched/mm/filemap_xip.c Â2009-03-31 15:25:53.000000000 +0200
> @@ -89,8 +89,8 @@ do_xip_mapping_read(struct address_space
> Â Â Â Â Â Â Â Â Â Â Â Â}
> Â Â Â Â Â Â Â Â}
> Â Â Â Â Â Â Â Ânr = nr - offset;
> - Â Â Â Â Â Â Â if (nr > len)
> - Â Â Â Â Â Â Â Â Â Â Â nr = len;
> + Â Â Â Â Â Â Â if (nr > len - copied)
> + Â Â Â Â Â Â Â Â Â Â Â nr = len - copied;
>
> Â Â Â Â Â Â Â Âerror = mapping->a_ops->get_xip_mem(mapping, index, 0,
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â&xip_mem, &xip_pfn);
>
> --
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-- 
Kinds regards,
Minchan Kim
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