Commit f4aacea2 authored by Kees Cook's avatar Kees Cook Committed by Linus Torvalds
Browse files

sysctl: allow for strict write position handling 



When writing to a sysctl string, each write, regardless of VFS position,
begins writing the string from the start.  This means the contents of
the last write to the sysctl controls the string contents instead of the
first:

  open("/proc/sys/kernel/modprobe", O_WRONLY)   = 1
  write(1, "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., 4096) = 4096
  write(1, "/bin/true", 9)                = 9
  close(1)                                = 0

  $ cat /proc/sys/kernel/modprobe
  /bin/true

Expected behaviour would be to have the sysctl be "AAAA..." capped at
maxlen (in this case KMOD_PATH_LEN: 256), instead of truncating to the
contents of the second write.  Similarly, multiple short writes would
not append to the sysctl.

The old behavior is unlike regular POSIX files enough that doing audits
of software that interact with sysctls can end up in unexpected or
dangerous situations.  For example, "as long as the input starts with a
trusted path" turns out to be an insufficient filter, as what must also
happen is for the input to be entirely contained in a single write
syscall -- not a common consideration, especially for high level tools.

This provides kernel.sysctl_writes_strict as a way to make this behavior
act in a less surprising manner for strings, and disallows non-zero file
position when writing numeric sysctls (similar to what is already done
when reading from non-zero file positions).  For now, the default (0) is
to warn about non-zero file position use, but retain the legacy
behavior.  Setting this to -1 disables the warning, and setting this to
1 enables the file position respecting behavior.

[akpm@linux-foundation.org: fix build]
[akpm@linux-foundation.org: move misplaced hunk, per Randy]
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2ca9bb45

Documentation/sysctl/kernel.txt

+21 −0
Original line number Diff line number Diff line
@@ -77,6 +77,7 @@ show up in /proc/sys/kernel: 
- shmmni

- stop-a                      [ SPARC only ]

- sysrq                       ==> Documentation/sysrq.txt

- sysctl_writes_strict

- tainted

- threads-max

- unknown_nmi_panic
@@ -762,6 +763,26 @@ without users and with a dead originative process will be destroyed. 


==============================================================



sysctl_writes_strict:



Control how file position affects the behavior of updating sysctl values

via the /proc/sys interface:



  -1 - Legacy per-write sysctl value handling, with no printk warnings.

       Each write syscall must fully contain the sysctl value to be

       written, and multiple writes on the same sysctl file descriptor

       will rewrite the sysctl value, regardless of file position.

   0 - (default) Same behavior as above, but warn about processes that

       perform writes to a sysctl file descriptor when the file position

       is not 0.

   1 - Respect file position when writing sysctl strings. Multiple writes

       will append to the sysctl value buffer. Anything past the max length

       of the sysctl value buffer will be ignored. Writes to numeric sysctl

       entries must always be at file position 0 and the value must be

       fully contained in the buffer sent in the write syscall.



==============================================================



tainted:



Non-zero if the kernel has been tainted.  Numeric values, which

kernel/sysctl.c

+67 −2
Original line number Diff line number Diff line
@@ -173,6 +173,13 @@ extern int no_unaligned_warning; 
#endif



#ifdef CONFIG_PROC_SYSCTL



#define SYSCTL_WRITES_LEGACY	-1

#define SYSCTL_WRITES_WARN	 0

#define SYSCTL_WRITES_STRICT	 1



static int sysctl_writes_strict = SYSCTL_WRITES_WARN;



static int proc_do_cad_pid(struct ctl_table *table, int write,

		  void __user *buffer, size_t *lenp, loff_t *ppos);

static int proc_taint(struct ctl_table *table, int write,
@@ -495,6 +502,15 @@ static struct ctl_table kern_table[] = { 
		.mode		= 0644,

		.proc_handler	= proc_taint,

	},

	{

		.procname	= "sysctl_writes_strict",

		.data		= &sysctl_writes_strict,

		.maxlen		= sizeof(int),

		.mode		= 0644,

		.proc_handler	= proc_dointvec_minmax,

		.extra1		= &neg_one,

		.extra2		= &one,

	},

#endif

#ifdef CONFIG_LATENCYTOP

	{
@@ -1717,8 +1733,20 @@ static int _proc_do_string(char *data, int maxlen, int write, 
	}



	if (write) {

		if (sysctl_writes_strict == SYSCTL_WRITES_STRICT) {

			/* Only continue writes not past the end of buffer. */

			len = strlen(data);

			if (len > maxlen - 1)

				len = maxlen - 1;



			if (*ppos > len)

				return 0;

			len = *ppos;

		} else {

			/* Start writing from beginning of buffer. */

			len = 0;

		}



		*ppos += *lenp;

		p = buffer;

		while ((p - buffer) < *lenp && len < maxlen - 1) {
@@ -1758,6 +1786,14 @@ static int _proc_do_string(char *data, int maxlen, int write, 
	return 0;

}



static void warn_sysctl_write(struct ctl_table *table)

{

	pr_warn_once("%s wrote to %s when file position was not 0!\n"

		"This will not be supported in the future. To silence this\n"

		"warning, set kernel.sysctl_writes_strict = -1\n",

		current->comm, table->procname);

}



/**

 * proc_dostring - read a string sysctl

 * @table: the sysctl table
@@ -1778,6 +1814,9 @@ static int _proc_do_string(char *data, int maxlen, int write, 
int proc_dostring(struct ctl_table *table, int write,

		  void __user *buffer, size_t *lenp, loff_t *ppos)

{

	if (write && *ppos && sysctl_writes_strict == SYSCTL_WRITES_WARN)

		warn_sysctl_write(table);



	return _proc_do_string((char *)(table->data), table->maxlen, write,

			       (char __user *)buffer, lenp, ppos);

}
@@ -1953,6 +1992,18 @@ static int __do_proc_dointvec(void *tbl_data, struct ctl_table *table, 
		conv = do_proc_dointvec_conv;



	if (write) {

		if (*ppos) {

			switch (sysctl_writes_strict) {

			case SYSCTL_WRITES_STRICT:

				goto out;

			case SYSCTL_WRITES_WARN:

				warn_sysctl_write(table);

				break;

			default:

				break;

			}

		}



		if (left > PAGE_SIZE - 1)

			left = PAGE_SIZE - 1;

		page = __get_free_page(GFP_TEMPORARY);
@@ -2010,6 +2061,7 @@ free: 
			return err ? : -EINVAL;

	}

	*lenp -= left;

out:

	*ppos += *lenp;

	return err;

}
@@ -2202,6 +2254,18 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int 
	left = *lenp;



	if (write) {

		if (*ppos) {

			switch (sysctl_writes_strict) {

			case SYSCTL_WRITES_STRICT:

				goto out;

			case SYSCTL_WRITES_WARN:

				warn_sysctl_write(table);

				break;

			default:

				break;

			}

		}



		if (left > PAGE_SIZE - 1)

			left = PAGE_SIZE - 1;

		page = __get_free_page(GFP_TEMPORARY);
@@ -2257,6 +2321,7 @@ free: 
			return err ? : -EINVAL;

	}

	*lenp -= left;

out:

	*ppos += *lenp;

	return err;

}

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