Messages in this thread Patch in this message | | | From | Heiko Carstens <> | Subject | [PATCH] fs/proc/kcore.c: use probe_kernel_read() instead of memcpy() | Date | Sat, 2 Dec 2017 14:27:39 +0100 |
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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 <heiko.carstens@de.ibm.com> --- 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
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