[PATCH] ash: fix use-after-free in bash pattern substitution

[Sören Tempel]

PING.

This has been applied at Alpine Linux downstream for the past few months
and we didn't encounter into any issues with this particular patch so far.

soeren at soeren-tempel.net wrote:
> From: Sören Tempel <soeren at soeren-tempel.net>
> 
> At Alpine Linux downstream, we were made aware of a segmentation fault
> occurring during string replacement in BusyBox ash [0]. Further
> debugging revealed that the segmentation fault occurs due to a
> use-after-free in BusyBox's bash pattern substitution implementation.
> Specially, the problem is that the repl variable (pointing to the
> replacement string) points to a value in the stack string. However, when
> accessing the repl pointer in Line 7350 it is possible that the stack
> has been moved since the last repl assignment due to the STPUTC
> invocations in Line 7317 and 7321 (since STPUTC may grow the stack via
> realloc(3)).
> 
> For this reason, the code in Line 7350 may access an unmapped memory
> region and therefore causes a segmentation fault if prior STPUTC
> invocations moved the stack via realloc(3). The valgrind output
> for this edge case looks as follows:
> 
> 	Invalid read of size 1
> 	   at 0x15D8DD: subevalvar (ash.c:7350)
> 	   by 0x15DC43: evalvar (ash.c:7666)
> 	   by 0x15B717: argstr (ash.c:6893)
> 	   by 0x15BAEC: expandarg (ash.c:8090)
> 	   by 0x15F4CC: evalcommand (ash.c:10429)
> 	   by 0x15B26C: evaltree (ash.c:9365)
> 	   by 0x15E4FC: cmdloop (ash.c:13569)
> 	   by 0x15FD8B: ash_main (ash.c:14748)
> 	   by 0x115BF2: run_applet_no_and_exit (appletlib.c:967)
> 	   by 0x115F16: run_applet_and_exit (appletlib.c:986)
> 	   by 0x115EF9: busybox_main (appletlib.c:917)
> 	   by 0x115EF9: run_applet_and_exit (appletlib.c:979)
> 	   by 0x115F8F: main (appletlib.c:1126)
> 	 Address 0x48b8646 is 2,054 bytes inside a block of size 4,776 free'd
> 	   at 0x48A6FC9: realloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
> 	   by 0x116E86: xrealloc (xfuncs_printf.c:61)
> 	   by 0x1565DB: growstackblock (ash.c:1736)
> 	   by 0x156EF7: growstackstr (ash.c:1775)
> 	   by 0x156F1A: _STPUTC (ash.c:1816)
> 	   by 0x15D843: subevalvar (ash.c:7317)
> 	   by 0x15DC43: evalvar (ash.c:7666)
> 	   by 0x15B717: argstr (ash.c:6893)
> 	   by 0x15BAEC: expandarg (ash.c:8090)
> 	   by 0x15F4CC: evalcommand (ash.c:10429)
> 	   by 0x15B26C: evaltree (ash.c:9365)
> 	   by 0x15E4FC: cmdloop (ash.c:13569)
> 
> A testcase for reproducing this edge case is provided in the downstream
> bug report [1]. This commit fixes the issue by reconstructing the repl
> pointer relative to stackblock() via strloc and slash_pos.
> 
> [0]: https://gitlab.alpinelinux.org/alpine/aports/-/issues/13469
> [1]: https://gitlab.alpinelinux.org/alpine/aports/-/issues/13469#note_210530
> 
> Signed-off-by: Sören Tempel <soeren at soeren-tempel.net>
> ---
> Discussion: I am not familiar with the ash code base. For this reason,
> it is presently unclear to me if there is a path where slash_pos < 0,
> STPUTC is invoked, and repl points to the stack. If so, handling for the
> case that slash_pos < 0 also needs to be added to the proposed path.
> Furthermore, I haven't tested this patch extensively so please review
> with extra care.
> 
>  shell/ash.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/shell/ash.c b/shell/ash.c
> index 55df54bd0..24f9a8270 100644
> --- a/shell/ash.c
> +++ b/shell/ash.c
> @@ -7346,6 +7346,12 @@ subevalvar(char *start, char *str, int strloc,
>  				idx = loc;
>  			}
>  
> +			// The STPUTC invocations above may resize and move the
> +			// stack via realloc(3). Since repl is a pointer into the
> +			// stack, we need to reconstruct it relative to stackblock().
> +			if (slash_pos >= 0)
> +				repl = (char *)stackblock() + strloc + slash_pos + 1;
> +
>  			//bb_error_msg("repl:'%s'", repl);
>  			for (loc = (char*)repl; *loc; loc++) {
>  				char *restart_detect = stackblock();
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