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

[Francis Laniel]

From: Alban Crequy <albancrequy@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Tested-by: Francis Laniel <flaniel@xxxxxxxxxxxxxxxxxxx>
---
 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