From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Subject: fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()
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...(?)
Link: http://lkml.kernel.org/r/20171202132739.99971-1-heiko.carstens@xxxxxxxxxx
Fixes: df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/proc/kcore.c | 18 +++++-------------
1 file changed, 5 insertions(+), 13 deletions(-)
diff -puN fs/proc/kcore.c~fs-proc-kcorec-use-probe_kernel_read-instead-of-memcpy fs/proc/kcore.c
--- a/fs/proc/kcore.c~fs-proc-kcorec-use-probe_kernel_read-instead-of-memcpy
+++ a/fs/proc/kcore.c
@@ -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 {
_