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exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC

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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.

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.

Link: https://lkml.kernel.org/r/874ksczru6.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-05-18 10:11:10 -05:00
parent 8b72ca9004
commit ccbb18b673
1 changed files with 38 additions and 42 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

80
fs/binfmt_script.c
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@ -16,14 +16,14 @@
#include <linux/fs.h>
static inline bool spacetab(char c) { return c == ' ' || c == '\t'; }
static inline char *next_non_spacetab(char *first, const char *last)
static inline const char *next_non_spacetab(const char *first, const char *last)
{
for (; first <= last; first++)
if (!spacetab(*first))
return first;
return NULL;
}
static inline char *next_terminator(char *first, const char *last)
static inline const char *next_terminator(const char *first, const char *last)
{
for (; first <= last; first++)
if (spacetab(*first) || !*first)
@ -33,8 +33,7 @@ static inline char *next_terminator(char *first, const char *last)
static int load_script(struct linux_binprm *bprm)
{
const char *i_arg, *i_name;
char *cp, *buf_end;
const char *i_name, *i_sep, *i_arg, *i_end, *buf_end;
struct file *file;
int retval;
@ -42,20 +41,6 @@ static int load_script(struct linux_binprm *bprm)
if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))
return -ENOEXEC;
/*
* 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;
/*
* This section handles parsing the #! line into separate
* interpreter path and argument strings. We must be careful
@ -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;