[RFC PATCH v1 1/2] maccess: fix writing offset in case of fault in strncpy_from_kernel_nofault()

From: Francis Laniel <flaniel@linux.microsoft.com>
To: linux-kernel@vger.kernel.org
Cc: Alban Crequy <alban.crequy@gmail.com>,
	Alban Crequy <albancrequy@microsoft.com>,
	Francis Laniel <flaniel@linux.microsoft.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Mykola Lysenko <mykolal@fb.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Shuah Khan <shuah@kernel.org>,
	linux-mm@kvack.org, bpf@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: [RFC PATCH v1 1/2] maccess: fix writing offset in case of fault in strncpy_from_kernel_nofault()
Date: Tue,  8 Nov 2022 20:52:06 +0100	[thread overview]
Message-ID: <20221108195211.214025-2-flaniel@linux.microsoft.com> (raw)
In-Reply-To: <20221108195211.214025-1-flaniel@linux.microsoft.com>

From: Alban Crequy <albancrequy@microsoft.com>

If a page fault occurs while copying the first byte, this function resets one
byte before dst.
As a consequence, an address could be modified and leaded to kernel crashes if
case the modified address was accessed later.

Signed-off-by: Alban Crequy <albancrequy@microsoft.com>
Tested-by: Francis Laniel <flaniel@linux.microsoft.com>
---
 mm/maccess.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/maccess.c b/mm/maccess.c
index 5f4d240f67ec..074f6b086671 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -97,7 +97,7 @@ long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
 	return src - unsafe_addr;
 Efault:
 	pagefault_enable();
-	dst[-1] = '\0';
+	dst[0] = '\0';
 	return -EFAULT;
 }

--
2.25.1

