Re: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC
From: Kees Cook <hidden>
Date: 2020-05-19 19:08:29
Also in:
linux-fsdevel, lkml
Possibly related (same subject, not in this thread)
- 2020-05-19 · Re: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC · Eric W. Biederman <hidden>
On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
The return code -ENOEXEC serves to tell search_binary_handler that it should continue searching for the binfmt to handle a given file. This makes return -ENOEXEC with a bprm->buf that is needed to continue the search problematic. The current binfmt_script manages to escape problems as it closes and clears bprm->file before return -ENOEXEC with bprm->buf modified. This prevents search_binary_handler from looping as it explicitly handles a NULL bprm->file. I plan on moving all of the bprm->file managment into fs/exec.c and out of the binary handlers so this will become a problem. Move closing bprm->file and the test for BINPRM_PATH_INACCESSIBLE down below the last return of -ENOEXEC. Introduce i_sep and i_end to track the end of the first argument and the end of the parameters respectively. Using those, constification of all char * pointers, and the helpers next_terminator and next_non_spacetab guarantee the parameter parsing will not modify bprm->buf.
I'm quite pleased this could be implemented using the existing helpers! It seems Linus and I were on the right track with these. :)
Only modify bprm->buf to terminate the strings i_arg and i_name with '\0' for passing to copy_strings_kernel. When replacing loops with next_non_spacetab and next_terminator care has been take that the logic of the parsing code (short of replacing characters by '\0') remains the same.
Ah, interesting. As in, bprm->buf must not be modified unless the binfmt handler is going to succeed. I think this requirement should be documented in the binfmt struct header file.
quoted hunk ↗ jump to hunk
[...]diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c index 8d718d8fd0fe..85e0ef86eb11 100644 --- a/fs/binfmt_script.c +++ b/fs/binfmt_script.c@@ -71,39 +56,48 @@ static int load_script(struct linux_binprm *bprm) * parse them on its own. */ buf_end = bprm->buf + sizeof(bprm->buf) - 1; - cp = strnchr(bprm->buf, sizeof(bprm->buf), '\n'); - if (!cp) { - cp = next_non_spacetab(bprm->buf + 2, buf_end); - if (!cp) + i_end = strnchr(bprm->buf, sizeof(bprm->buf), '\n'); + if (!i_end) { + i_end = next_non_spacetab(bprm->buf + 2, buf_end); + if (!i_end) return -ENOEXEC; /* Entire buf is spaces/tabs */ /* * If there is no later space/tab/NUL we must assume the * interpreter path is truncated. */ - if (!next_terminator(cp, buf_end)) + if (!next_terminator(i_end, buf_end)) return -ENOEXEC; - cp = buf_end; + i_end = buf_end; } - /* NUL-terminate the buffer and any trailing spaces/tabs. */ - *cp = '\0'; - while (cp > bprm->buf) { - cp--; - if ((*cp == ' ') || (*cp == '\t')) - *cp = '\0'; - else - break; - } - for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++); - if (*cp == '\0') + /* Trim any trailing spaces/tabs from i_end */ + while (spacetab(i_end[-1])) + i_end--; + + /* Skip over leading spaces/tabs */ + i_name = next_non_spacetab(bprm->buf+2, i_end); + if (!i_name || (i_name == i_end)) return -ENOEXEC; /* No interpreter name found */ - i_name = cp; + + /* Is there an optional argument? */ i_arg = NULL; - for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++) - /* nothing */ ; - while ((*cp == ' ') || (*cp == '\t')) - *cp++ = '\0'; - if (*cp) - i_arg = cp; + i_sep = next_terminator(i_name, i_end); + if (i_sep && (*i_sep != '\0')) + i_arg = next_non_spacetab(i_sep, i_end); + + /* + * If the script filename will be inaccessible after exec, typically + * because it is a "/dev/fd/<fd>/.." path against an O_CLOEXEC fd, give + * up now (on the assumption that the interpreter will want to load + * this file). + */ + if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE) + return -ENOENT; + + /* Release since we are not mapping a binary into memory. */ + allow_write_access(bprm->file); + fput(bprm->file); + bprm->file = NULL; + /* * OK, we've parsed out the interpreter name and * (optional) argument.@@ -121,7 +115,9 @@ static int load_script(struct linux_binprm *bprm) if (retval < 0) return retval; bprm->argc++; + *((char *)i_end) = '\0'; if (i_arg) { + *((char *)i_sep) = '\0'; retval = copy_strings_kernel(1, &i_arg, bprm); if (retval < 0) return retval;
I think this is all correct, though I'm always suspicious of my visual inspection of string parsers. ;) I had a worry the \n was not handled correctly in some case. I.e. before any \n was converted into \0, and so next_terminator() didn't need to consider \n separately. (next_non_spacetab() doesn't care since \n and \0 are both not ' ' nor '\t'.) For next_terminator(), though, I was worried there was a case where *i_end == '\n', and next_terminator() will return NULL instead of "last" due to *last being '\n' instead of '\0', causing a problem, but you're using the adjusted i_end so I think it's correct. And you've handled i_name == i_end. I will see if I can find my testing scripts I used when commit b5372fe5dc84 originally landed to double-check... until then: Reviewed-by: Kees Cook <redacted> -- Kees Cook