Commit d0290bc2 authored Feb 06, 2018 by Heiko Carstens Committed by Linus Torvalds Feb 06, 2018

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

Commit df04abfd ("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 df04abfd ("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...(?)

Link: http://lkml.kernel.org/r/20171202132739.99971-1-heiko.carstens@de.ibm.com


Fixes: df04abfd ("fs/proc/kcore.c: Add bounce buffer for ktext data")
Fixes: f5509cc1 ("mm: Hardened usercopy")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 171ef917

fs/proc/kcore.c

+5 −13

Original line number	Diff line number	Diff line
		@@ -512,23 +512,15 @@ read_kcore(struct file file, char __user buffer, size_t buflen, loff_t *fpos)
		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 {

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