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authorKees Cook <keescook@chromium.org>2014-06-06 14:37:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-06 16:08:13 -0700
commitf4aacea2f5d1a5f7e3154e967d70cf3f711bcd61 (patch)
tree6706ce16774c72bcbb33e4872b4913731349cb13 /kernel/sysctl.c
parent2ca9bb456ada8bcbdc8f77f8fc78207653bbaa92 (diff)
downloadlinux-sh-f4aacea2f5d1a5f7e3154e967d70cf3f711bcd61.tar.gz
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: Kees Cook <keescook@chromium.org> Cc: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/sysctl.c')
-rw-r--r--kernel/sysctl.c69
1 files changed, 67 insertions, 2 deletions
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index ac6847feaa83..7a910b9081e8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -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) {
- /* Start writing from beginning of buffer. */
- len = 0;
+ 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;
}