[Rd] tools::parseLatex() crashes on "\\verb{}"

[Ivan Krylov]

On Thu, 20 Jul 2023 21:41:44 +0200
Antoine Fabri <antoine.fabri using gmail.com> wrote:

> tools::parseLatex("\\verb{hello}")
> # crashes the session

Looking at the source [*], this seems to be happening because
parseLatex expects the \verb macro to use the same character as the
delimiter on both sides:

tools::parseLatex('\\verb!hello!')
# \verb!hello!

What the loop doesn't have is a check for EOF, which leads TEXT_PUSH()
to increase the temporary buffer exponentially until unsigned int
nstext overflows and results in a 0-byte allocation, which is then
overrun, corrupting the heap. Any other unterminated \verb!... would
have caused the same crash.

Here's a patch that prevents this particular crash:

--- src/library/tools/src/gramLatex.y	(revision 84714)
+++ src/library/tools/src/gramLatex.y	(working copy)
@@ -846,8 +846,8 @@
     
     TEXT_PUSH('\\'); TEXT_PUSH('v'); TEXT_PUSH('e'); TEXT_PUSH('r'); TEXT_PUSH('b');
     TEXT_PUSH(c);
-    while ((c = xxgetc()) != delim) TEXT_PUSH(c);
-    TEXT_PUSH(c);
+    while (((c = xxgetc()) != delim) && c != R_EOF) TEXT_PUSH(c);
+    if (c != R_EOF) TEXT_PUSH(c);
     
     PRESERVE_SV(yylval = mkString2(stext, bp - stext));
     if(stext != st0) free(stext);

This seems to have been the only remaining while loop in gramLatex.y
that didn't check for R_EOF.

More correctness work is needed: mkMarkup() should avoid calling
mkVerb(R_EOF) when running tools::parseLatex('\\verb'), since otherwise
0xFF becomes a part of the resulting text. All declarations of unsigned
int nstext should probably be replaced by size_t nstext... but then
we'd have an annoying visit from the OOM killer instead of a much faster
crash in case of a runaway TEXT_PUSH(), and nobody expects to parse 4
GB of LaTeX source anyway. TEXT_PUSH() probably needs an integer
overflow check and to free the temporary buffer before calling error().

-- 
Best regards,
Ivan

[*]
https://github.com/r-devel/r-svn/blob/f145419cc4dae162719206a61a29082adff2043d/src/library/tools/src/gramLatex.y#L845-L850