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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Alban Crequy <albancrequy@linux.microsoft.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Francis Laniel <flaniel@linux.microsoft.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH 5.15 145/181] maccess: Fix writing offset in case of fault in strncpy_from_kernel_nofault()
Date: Wed, 23 Nov 2022 09:51:48 +0100
Message-Id: <20221123084608.624504876@linuxfoundation.org>
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From: Alban Crequy <albancrequy@linux.microsoft.com>

commit 8678ea06852cd1f819b870c773d43df888d15d46 upstream.

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.

Fixes: b58294ead14c ("maccess: allow architectures to provide kernel probing directly")
Signed-off-by: Alban Crequy <albancrequy@linux.microsoft.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Francis Laniel <flaniel@linux.microsoft.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org> [5.8]
Link: https://lore.kernel.org/bpf/20221110085614.111213-2-albancrequy@linux.microsoft.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/maccess.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -99,7 +99,7 @@ long strncpy_from_kernel_nofault(char *d
 	return src - unsafe_addr;
 Efault:
 	pagefault_enable();
-	dst[-1] = '\0';
+	dst[0] = '\0';
 	return -EFAULT;
 }
 #else /* HAVE_GET_KERNEL_NOFAULT */