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From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Baoquan He <bhe@redhat.com>, Uladzislau Rezki <urezki@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Liu Shixin <liushixin2@huawei.com>,
        Jiri Olsa <jolsa@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Lorenzo Stoakes <lstoakes@gmail.com>
Subject: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data
Date:   Thu, 23 Mar 2023 10:15:16 +0000
Message-Id: <fd39b0bfa7edc76d360def7d034baaee71d90158.1679566220.git.lstoakes@gmail.com>
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Commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
introduced the use of a bounce buffer to retrieve kernel text data for
/proc/kcore in order to avoid failures arising from hardened user copies
enabled by CONFIG_HARDENED_USERCOPY in check_kernel_text_object().

We can avoid doing this if instead of copy_to_user() we use _copy_to_user()
which bypasses the hardening check. This is more efficient than using a
bounce buffer and simplifies the code.

We do so as part an overall effort to eliminate bounce buffer usage in the
function with an eye to converting it an iterator read.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 fs/proc/kcore.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 71157ee35c1a..556f310d6aa4 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -541,19 +541,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 		case KCORE_VMEMMAP:
 		case KCORE_TEXT:
 			/*
-			 * Using bounce buffer to bypass the
-			 * hardened user copy kernel text checks.
+			 * We use _copy_to_user() to bypass usermode hardening
+			 * which would otherwise prevent this operation.
 			 */
-			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
-				if (clear_user(buffer, tsz)) {
-					ret = -EFAULT;
-					goto out;
-				}
-			} else {
-				if (copy_to_user(buffer, buf, tsz)) {
-					ret = -EFAULT;
-					goto out;
-				}
+			if (_copy_to_user(buffer, (char *)start, tsz)) {
+				ret = -EFAULT;
+				goto out;
 			}
 			break;
 		default:
-- 
2.39.2