4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Heiko Carstens heiko.carstens@de.ibm.com
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...(?)
Link: http://lkml.kernel.org/r/20171202132739.99971-1-heiko.carstens@de.ibm.com Fixes: df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data") Fixes: f5509cc18daa ("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 Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/proc/kcore.c | 18 +++++------------- 1 file changed, 5 insertions(+), 13 deletions(-)
--- a/fs/proc/kcore.c +++ b/fs/proc/kcore.c @@ -507,23 +507,15 @@ 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 {