[PATCH] ima: bound line scan in ima_read_policy() to fix OOB read

[Fabrice Derepas]

ima_read_policy() loads a policy file with kernel_read_file_from_path()
and splits it into lines with

	while (size > 0 && (p = strsep(&datap, "\n")))

kernel_read_file() allocates the destination with vmalloc(i_size) --
exactly i_size bytes, and writes no NUL terminator. strsep()'s scan for
the next '\n' is not bounded by @size, so when the last line has no
trailing newline the scan runs off the end of the buffer (CWE-125). When
i_size is a multiple of PAGE_SIZE the allocation has no slack and the
read walks into the vmalloc guard page and faults.

Reproduced under KASAN in a VM: writing the path of a page-aligned
policy file with no trailing newline to <securityfs>/ima/policy oopses:

  BUG: unable to handle page fault for address: ffffc90000032000
  #PF: supervisor read access in kernel mode
  RIP: 0010:strsep+0x7a/0xd0
  Call Trace:
   ima_write_policy+0x1f4/0x260
   vfs_write+0x16a/0x6f0
   ksys_write+0xcb/0x160
   do_syscall_64+0xe0/0x5a0

This requires CAP_MAC_ADMIN (the policy file is mode 0200), but a policy
file that does not end in a newline is an ordinary, non-malicious
condition, so a legitimate policy load can crash the kernel.

Walk the buffer with memchr() bounded by the remaining size instead of
strsep(): terminate each line in place at its newline, and parse a
NUL-terminated copy of a final line that has none. The explicit per-line
accounting replaces the old "size -= rc" step, whose off-by-one
(ima_parse_add_rule() returns strlen() + 1) made a trailing line without a
newline fail with -EINVAL; such a policy now loads. The loop now consumes
the buffer exactly, so the trailing "if (size) return -EINVAL" is dropped.

Fixes: 7429b092811f ("ima: load policy using path")
Assisted-by: copilot-cli:claude-opus-4-6 frama-c
Signed-off-by: Fabrice Derepas <fabrice.derepas at canonical.com>
---
Tested under KASAN (CONFIG_KASAN_GENERIC + CONFIG_KASAN_VMALLOC) in QEMU,
loading a policy via "echo /path > <securityfs>/ima/policy":

  - page-aligned file, no trailing newline: unpatched -> guard-page oops
    in strsep()/ima_read_policy() (trace above); patched -> no fault, the
    load fails cleanly with -EINVAL on the (garbage) content.
  - valid policy with a trailing newline: loads before and after.
  - valid rule with no trailing newline: unpatched -> -EINVAL (the size
    underflow); patched -> loads.

lib/string.o is not KASAN-instrumented, so the over-read is caught by the
vmalloc guard page rather than a shadow report; the confirmation is the
page-fault oops with strsep()/ima_write_policy() in the trace.

 security/integrity/ima/ima_fs.c | 46 ++++++++++++++++++++++++++-------
 1 file changed, 36 insertions(+), 10 deletions(-)

diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
index 174a94740..7b530b130 100644
--- a/security/integrity/ima/ima_fs.c
+++ b/security/integrity/ima/ima_fs.c
@@ -526,12 +526,10 @@ static const struct file_operations ima_ascii_measurements_staged_ops = {
 static ssize_t ima_read_policy(char *path)
 {
 	void *data = NULL;
-	char *datap;
-	size_t size;
+	char *datap, *eol, *p;
+	size_t size, linelen;
 	int rc, pathlen = strlen(path);
 
-	char *p;
-
 	/* remove \n */
 	datap = path;
 	strsep(&datap, "\n");
@@ -546,21 +544,49 @@ static ssize_t ima_read_policy(char *path)
 	rc = 0;
 
 	datap = data;
-	while (size > 0 && (p = strsep(&datap, "\n"))) {
+	while (size > 0) {
+		eol = memchr(datap, '\n', size);
+		linelen = eol ? (size_t)(eol - datap) : size;
+
+		if (eol) {
+			/* NUL-terminate the line in place, within bounds. */
+			*eol = '\0';
+			p = datap;
+		} else {
+			/*
+			 * kernel_read_file_from_path() does not NUL-terminate
+			 * the buffer, and it may be exactly i_size bytes long,
+			 * so a string walk off the end is possible.  The final
+			 * line without a trailing newline has no room for a
+			 * terminator; parse a terminated copy instead.
+			 */
+			p = kmemdup_nul(datap, linelen, GFP_KERNEL);
+			if (!p) {
+				rc = -ENOMEM;
+				break;
+			}
+		}
+
 		pr_debug("rule: %s\n", p);
 		rc = ima_parse_add_rule(p);
+		if (!eol)
+			kfree(p);
 		if (rc < 0)
 			break;
-		size -= rc;
+		rc = 0;
+
+		datap += linelen;
+		size -= linelen;
+		if (eol) {
+			datap++;	/* skip the newline */
+			size--;
+		}
 	}
 
 	vfree(data);
 	if (rc < 0)
 		return rc;
-	else if (size)
-		return -EINVAL;
-	else
-		return pathlen;
+	return pathlen;
 }
 
 static ssize_t ima_write_policy(struct file *file, const char __user *buf,

base-commit: d58772d8520c7ef247c4b95c9bd76d3a25da9ff5
-- 
2.53.0