linux-kernel - [PATCH 3.16 050/204] KEYS: fix writing past end of user-supplied buffer in keyring

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Date:   Thu, 28 Dec 2017 17:05:44 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David Howells" <dhowells@...hat.com>,
        "Eric Biggers" <ebiggers@...gle.com>
Subject: [PATCH 3.16 050/204] KEYS: fix writing past end of user-supplied
 buffer in keyring_read()

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@...gle.com>

commit e645016abc803dafc75e4b8f6e4118f088900ffb upstream.

Userspace can call keyctl_read() on a keyring to get the list of IDs of
keys in the keyring.  But if the user-supplied buffer is too small, the
kernel would write the full list anyway --- which will corrupt whatever
userspace memory happened to be past the end of the buffer.  Fix it by
only filling the space that is available.

Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 security/keys/keyring.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -405,7 +405,7 @@ static void keyring_describe(const struc
 }
 
 struct keyring_read_iterator_context {
-	size_t			qty;
+	size_t			buflen;
 	size_t			count;
 	key_serial_t __user	*buffer;
 };
@@ -417,9 +417,9 @@ static int keyring_read_iterator(const v
 	int ret;
 
 	kenter("{%s,%d},,{%zu/%zu}",
-	       key->type->name, key->serial, ctx->count, ctx->qty);
+	       key->type->name, key->serial, ctx->count, ctx->buflen);
 
-	if (ctx->count >= ctx->qty)
+	if (ctx->count >= ctx->buflen)
 		return 1;
 
 	ret = put_user(key->serial, ctx->buffer);
@@ -454,16 +454,12 @@ static long keyring_read(const struct ke
 		return 0;
 
 	/* Calculate how much data we could return */
-	ctx.qty = nr_keys * sizeof(key_serial_t);
-
 	if (!buffer || !buflen)
-		return ctx.qty;
-
-	if (buflen > ctx.qty)
-		ctx.qty = buflen;
+		return nr_keys * sizeof(key_serial_t);
 
 	/* Copy the IDs of the subscribed keys into the buffer */
 	ctx.buffer = (key_serial_t __user *)buffer;
+	ctx.buflen = buflen;
 	ctx.count = 0;
 	ret = assoc_array_iterate(&keyring->keys, keyring_read_iterator, &ctx);
 	if (ret < 0) {