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Fix use-after-free in error message construction #217

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Fix use-after-free in error message construction #217

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10 changes: 6 additions & 4 deletions src/openssl.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1487,15 +1487,17 @@ static const char *auxL_pusherror(lua_State *L, int error, const char *fun) {
file = path;
}

ERR_clear_error();

ERR_error_string_n(code, txt, sizeof txt);

if (fun) {
return lua_pushfstring(L, "%s: %s:%d:%s", fun, file, line, txt);
lua_pushfstring(L, "%s: %s:%d:%s", fun, file, line, txt);
} else {
return lua_pushfstring(L, "%s:%d:%s", file, line, txt);
lua_pushfstring(L, "%s:%d:%s", file, line, txt);
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lua_pushfstring can fail (i.e. longjmp out). We need to clear the OpenSSL error before calling it.

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I'd rather take a potential longjmp (due to memory allocation failure) over consistently using released memory (the current situation).

Nevertheless, I'm happy to amend the patch. But what do you suggest? Allocating a temporary buffer to hold the string also has the potential to fail if lua_pushfstring() may fail. Then what?

I haven't dug deeply into the OpenSSL error handling API docs, but as far as I can tell, if we don't clear the error it just remains on the stack, it doesn't actually "leak".

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We already have the temporary (stack-allocated) buffer txt. ERR_error_string_n copies into that.
So the patch really only needs to swap the order of ERR_clear_error and ERR_error_string_n

}

ERR_clear_error();

return 1;
#if HAVE_DLADDR
} else if (error == auxL_EDYLD) {
const char *const fmt = (fun)? "%s: %s" : "%.0s%s";
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