Bug in Dash's unquoting of backslashes within backquoted strings

Bug in Dash's unquoting of backslashes within backquoted strings

[Matt Whitlock]

A minimal example:

: `: "\\\\
\\$(bug)"`

That's one command, containing a newline character, embedded in a 
double-quoted string, embedded in a backquoted string. The command is an 
invocation of the : (colon) built-in, passing arguments obtained by word 
splitting the result of the backquoted command substitution.

The Open Group says: "Within the backquoted style of command substitution, 
backslash shall retain its literal meaning, except when followed by: '$', 
'`', or '\' (dollar sign, backquote, backslash)." Thus, each pair of 
backslashes between the backquotes is to be reduced to a single backslash.

Thus, the subcommand to be executed is:

: "\\
\$(bug)"

This subcommand is an invocation of the : (colon) built-in command, passing 
a single argument obtained by performing quote removal on the double-quoted 
string. After quote removal, the resulting argument consists of these eight 
characters: backslash, newline, dollar sign, open parenthesis, b, u, g, 
close parenthesis.

If the above subcommand is entered directly at the Dash command line, all 
is well. However, when it appears inside a backquoted subcommand (with the 
backslash characters being appropriately escaped), such as given at the top 
of this report, then Dash processes it incorrectly:

/bin/sh: 1: bug: not found

Dash apparently skips over the backslash immediately following the newline 
embedded in the double-quoted string. Thus, Dash sees the dollar sign as 
introducing a command substitution rather than as a literal character.

This does not happen if the backslashes preceding the embedded newline are 
absent. It also does not happen if any other character, including linear 
whitespace, is inserted immediately after the embedded newline.

Both Bash and Busybox Ash get it right. Dash is non-conformant.

This bug has potentially grave security implications since Dash is 
interpreting a string literal as though it were a command and executing it. 
If the word "bug" in my examples above had been "rm -rf /" instead, the 
results would have been catastrophic.

Re: Bug in Dash's unquoting of backslashes within backquoted strings

[Ron Yorston]

Matt Whitlock wrote:
>A minimal example:
>
>: `: "\\\\
>\\$(bug)"`

>However, when it appears inside a backquoted subcommand (with the 
>backslash characters being appropriately escaped), such as given at the top 
>of this report, then Dash processes it incorrectly:
>
>/bin/sh: 1: bug: not found

This seems to have been introduced by commit 6bbc71d (parser: use
pgetc_eatbnl() in more places).  Reverting the following part of the
commit makes the problem go away:

            case '\\':
-                                if ((pc = pgetc()) == '\n') {
-                   nlprompt();
-                   /*
-                    * If eating a newline, avoid putting
-                    * the newline into the new character
-                    * stream (via the STPUTC after the
-                    * switch).
-                    */
-                   continue;
-               }
+                                pc = pgetc_eatbnl();

Ron

Re: Bug in Dash's unquoting of backslashes within backquoted strings

[Ron Yorston]

Ron Yorston wrote:
>This seems to have been introduced by commit 6bbc71d (parser: use
>pgetc_eatbnl() in more places).  Reverting the following part of the
>commit makes the problem go away:
>
>            case '\\':
>-                                if ((pc = pgetc()) == '\n') {
>-                   nlprompt();
>-                   /*
>-                    * If eating a newline, avoid putting
>-                    * the newline into the new character
>-                    * stream (via the STPUTC after the
>-                    * switch).
>-                    */
>-                   continue;
>-               }
>+                                pc = pgetc_eatbnl();

Alternatively I see that BusyBox ash did this:

            case '\\':
-               pc = pgetc();
-               if (pc == '\n') {
-                   nlprompt();
-                   /*
-                    * If eating a newline, avoid putting
-                    * the newline into the new character
-                    * stream (via the STPUTC after the
-                    * switch).
-                    */
-                   continue;
-               }
+               pc = pgetc(); /* or pgetc_eatbnl()? why (example)? */

Ron

Re: Bug in Dash's unquoting of backslashes within backquoted strings

[Harald van Dijk]

On 21/05/2020 22:38, Ron Yorston wrote:
> Matt Whitlock wrote:
>> A minimal example:
>>
>> : `: "\\\\
>> \\$(bug)"`
> 
>> However, when it appears inside a backquoted subcommand (with the
>> backslash characters being appropriately escaped), such as given at the top
>> of this report, then Dash processes it incorrectly:
>>
>> /bin/sh: 1: bug: not found
> 
> This seems to have been introduced by commit 6bbc71d (parser: use
> pgetc_eatbnl() in more places).  Reverting the following part of the
> commit makes the problem go away:

Yes, that commit (a patch I had submitted) was buggy and the part that 
you suggest reverting should be.

Cheers,
Harald van Dijk

parser: Fix double-backslash nl in old-style command sub

[Herbert Xu]

Ron Yorston <rmy@frippery.org> wrote:
>
> Alternatively I see that BusyBox ash did this:
> 
>            case '\\':
> -               pc = pgetc();
> -               if (pc == '\n') {
> -                   nlprompt();
> -                   /*
> -                    * If eating a newline, avoid putting
> -                    * the newline into the new character
> -                    * stream (via the STPUTC after the
> -                    * switch).
> -                    */
> -                   continue;
> -               }
> +               pc = pgetc(); /* or pgetc_eatbnl()? why (example)? */

Yes this is the correct fix.

---8<---
When handling backslashes within an old-style command substitution,
we should not call pgetc_eatbnl because that would treat the next
backslash character as another escape character if it was then
followed by a new-line.

This patch fixes it by calling pgetc.

Reported-by: Matt Whitlock <dash@mattwhitlock.name>
Fixes: 6bbc71d84bea ("parser: use pgetc_eatbnl() in more places")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/src/parser.c b/src/parser.c
index 3131045..03c103b 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -1393,7 +1393,7 @@ parsebackq: {
 				goto done;
 
 			case '\\':
-                                pc = pgetc_eatbnl();
+                                pc = pgetc();
                                 if (pc != '\\' && pc != '`' && pc != '$'
                                     && (!synstack->dblquote || pc != '"'))
                                         STPUTC('\\', pout);
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt