From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001

From: Peter Jones <pjones@redhat.com>

Date: Wed, 15 Apr 2020 15:45:02 -0400

Subject: [PATCH] yylex: Make lexer fatal errors actually be fatal

When presented with a command that can't be tokenized to anything

smaller than YYLMAX characters, the parser calls YY_FATAL_ERROR(errmsg),

expecting that will stop further processing, as such:

  #define YY_DO_BEFORE_ACTION \

        yyg->yytext_ptr = yy_bp; \

        yyleng = (int) (yy_cp - yy_bp); \

        yyg->yy_hold_char = *yy_cp; \

        *yy_cp = '\0'; \

        if ( yyleng >= YYLMAX ) \

                YY_FATAL_ERROR( "token too large, exceeds YYLMAX" ); \

        yy_flex_strncpy( yytext, yyg->yytext_ptr, yyleng + 1 , yyscanner); \

        yyg->yy_c_buf_p = yy_cp;

The code flex generates expects that YY_FATAL_ERROR() will either return

for it or do some form of longjmp(), or handle the error in some way at

least, and so the strncpy() call isn't in an "else" clause, and thus if

YY_FATAL_ERROR() is *not* actually fatal, it does the call with the

questionable limit, and predictable results ensue.

Unfortunately, our implementation of YY_FATAL_ERROR() is:

   #define YY_FATAL_ERROR(msg)                     \

     do {                                          \

       grub_printf (_("fatal error: %s\n"), _(msg));     \

     } while (0)

The same pattern exists in yyless(), and similar problems exist in users

of YY_INPUT(), several places in the main parsing loop,

yy_get_next_buffer(), yy_load_buffer_state(), yyensure_buffer_stack,

yy_scan_buffer(), etc.

All of these callers expect YY_FATAL_ERROR() to actually be fatal, and

the things they do if it returns after calling it are wildly unsafe.

Fixes: CVE-2020-10713

Signed-off-by: Peter Jones <pjones@redhat.com>

Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Upstream-commit-id: 926df817dc8

---

 grub-core/script/yylex.l | 4 ++--

 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/grub-core/script/yylex.l b/grub-core/script/yylex.l

index 7b44c37b76f..b7203c82309 100644

--- a/grub-core/script/yylex.l

+++ b/grub-core/script/yylex.l

@@ -37,11 +37,11 @@

 /* 

  * As we don't have access to yyscanner, we cannot do much except to

- * print the fatal error.

+ * print the fatal error and exit.

  */

 #define YY_FATAL_ERROR(msg)                     \

   do {                                          \

-    grub_printf (_("fatal error: %s\n"), _(msg));     \

+    grub_fatal (_("fatal error: %s\n"), _(msg));\

   } while (0)

 #define COPY(str, hint)                         \