maccess: Fix writing offset in case of fault in strncpy_from_kernel_nofault()

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

diff --git a/mm/maccess.c b/mm/maccess.c
index 5f4d240f67ecc0cae5c0900ae6e9545185847e0b..074f6b086671ebd25c59cabf84591344e6248aef 100644 (file)
--- 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;
 }