Patch "fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fs-proc-kcore.c-use-probe_kernel_read-instead-of-memcpy.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Heiko Carstens <[email protected]>
Date: Tue, 6 Feb 2018 15:37:13 -0800
Subject: fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()

From: Heiko Carstens <[email protected]>

commit d0290bc20d4739b7a900ae37eb5d4cc3be2b393f upstream.

Commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data") added a bounce buffer to avoid hardened usercopy checks. Copying to the bounce buffer was implemented with a simple memcpy() assuming that it is always valid to read from kernel memory iff the kern_addr_valid() check passed.

A simple, but pointless, test case like "dd if=/proc/kcore of=/dev/null" now can easily crash the kernel, since the former execption handling on invalid kernel addresses now doesn't work anymore.

Also adding a kern_addr_valid() implementation wouldn't help here. Most architectures simply return 1 here, while a couple implemented a page table walk to figure out if something is mapped at the address in question.

With DEBUG_PAGEALLOC active mappings are established and removed all the time, so that relying on the result of kern_addr_valid() before executing the memcpy() also doesn't work.

Therefore simply use probe_kernel_read() to copy to the bounce buffer. This also allows to simplify read_kcore().

At least on s390 this fixes the observed crashes and doesn't introduce warnings that were removed with df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data"), even though the generic probe_kernel_read() implementation uses uaccess functions.

While looking into this I'm also wondering if kern_addr_valid() could be completely removed...(?)

---
 fs/proc/kcore.c |   18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

@@ -512,23 +512,15 @@ read_kcore(struct file *file, char __use
 				return -EFAULT;
 		} else {
 			if (kern_addr_valid(start)) {
-				unsigned long n;
-
 				/*
 				 * Using bounce buffer to bypass the
 				 * hardened user copy kernel text checks.
 				 */
-				memcpy(buf, (char *) start, tsz);
-				n = copy_to_user(buffer, buf, tsz);
-				/*
-				 * We cannot distinguish between fault on source
-				 * and fault on destination. When this happens
-				 * we clear too and hope it will trigger the
-				 * EFAULT again.
-				 */
-				if (n) { 
-					if (clear_user(buffer + tsz - n,
-								n))
+				if (probe_kernel_read(buf, (void *) start, tsz)) {
+					if (clear_user(buffer, tsz))
+						return -EFAULT;
+				} else {
+					if (copy_to_user(buffer, buf, tsz))
 						return -EFAULT;
 				}
 			} else {

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/fs-proc-kcore.c-use-probe_kernel_read-instead-of-memcpy.patch