git 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...(?)
Fixes: df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Heiko Carstens <[email protected]>
---
fs/proc/kcore.c | 18 +++++-------------
1 file changed, 5 insertions(+), 13 deletions(-)
diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 4bc85cb8be6a..e8a93bc8285d 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -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 {
--
2.13.5
