linux-kernel - Re: [PATCH] perf buildid: fix off-by-one in write

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Date:	Tue, 19 Apr 2016 10:38:41 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf buildid: fix off-by-one in write_buildid()

Em Tue, Apr 19, 2016 at 11:17:27AM +0300, Andrey Ryabinin escreveu:
> write_buildid() increments 'name_len' with intention to take into account
> trailing zero byte. However, 'name_len' was already incremented in
> machine__write_buildid_table() before.
> So this leads to out-of-bounds read in do_write():

Could we keep the assumptions that for a string 's' the length is
strlen(s) and that when we want to write a string _with_ its trailing
'\0' we should use strlen(s) + 1?

I.e. I propose this patch instead, ok?

diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
index 0573c2ec861d..b6ecf87bc3e3 100644
--- a/tools/perf/util/build-id.c
+++ b/tools/perf/util/build-id.c
@@ -261,14 +261,14 @@ static int machine__write_buildid_table(struct machine *machine, int fd)
 
 		if (dso__is_vdso(pos)) {
 			name = pos->short_name;
-			name_len = pos->short_name_len + 1;
+			name_len = pos->short_name_len;
 		} else if (dso__is_kcore(pos)) {
 			machine__mmap_name(machine, nm, sizeof(nm));
 			name = nm;
-			name_len = strlen(nm) + 1;
+			name_len = strlen(nm);
 		} else {
 			name = pos->long_name;
-			name_len = pos->long_name_len + 1;
+			name_len = pos->long_name_len;
 		}
 
 		in_kernel = pos->kernel ||
 
> $ ./perf record sleep 0
> [ perf record: Woken up 1 times to write data ]
> =================================================================
> ==15899==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000099fc92 at pc 0x7f1aa9c7eab5 bp 0x7fff940f84d0 sp 0x7fff940f7c78
> READ of size 19 at 0x00000099fc92 thread T0
>     #0 0x7f1aa9c7eab4  (/usr/lib/gcc/x86_64-pc-linux-gnu/5.3.0/libasan.so.2+0x44ab4)
>     #1 0x649c5b in do_write util/header.c:67
>     #2 0x649c5b in write_padded util/header.c:82
>     #3 0x57e8bc in write_buildid util/build-id.c:239
>     #4 0x57e8bc in machine__write_buildid_table util/build-id.c:278
> ...
> 
> 0x00000099fc92 is located 0 bytes to the right of global variable '*.LC99' defined in 'util/symbol.c' (0x99fc80) of size 18
>   '*.LC99' is ascii string '[kernel.kallsyms]'
> ...
> 
> Shadow bytes around the buggy address:
>   0x00008012bf80: f9 f9 f9 f9 00 00 00 00 00 00 03 f9 f9 f9 f9 f9
> =>0x00008012bf90: 00 00[02]f9 f9 f9 f9 f9 00 00 00 00 00 05 f9 f9
>   0x00008012bfa0: f9 f9 f9 f9 00 03 f9 f9 f9 f9 f9 f9 00 00 00 00
> 
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> ---
>  tools/perf/util/build-id.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/build-id.c b/tools/perf/util/build-id.c
> index 0573c2e..a1ff68b 100644
> --- a/tools/perf/util/build-id.c
> +++ b/tools/perf/util/build-id.c
> @@ -235,7 +235,7 @@ static int write_buildid(const char *name, size_t name_len, u8 *build_id,
>  	if (err < 0)
>  		return err;
>  
> -	return write_padded(fd, name, name_len + 1, len);
> +	return write_padded(fd, name, name_len, len);
>  }
>  
>  static int machine__write_buildid_table(struct machine *machine, int fd)
> -- 
> 2.7.3