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Subject[PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC

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.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/binfmt_script.c | 80 ++++++++++++++++++++++------------------------
1 file changed, 38 insertions(+), 42 deletions(-)

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
@@ -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;
--
2.25.0
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