linux-kernel - [PATCH] nvmem: core: Fix OOB read for bit offsets of more than one byte

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Message-Id: <20250901-nvmem-read-oob-bit-offset-v1-1-b610e18cdd3c@jannau.net>
Date: Mon, 01 Sep 2025 09:29:43 +0200
From: Janne Grunau <j@...nau.net>
To: Srinivas Kandagatla <srini@...nel.org>, 
 Dmitry Baryshkov <lumag@...nel.org>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, Janne Grunau <j@...nau.net>
Subject: [PATCH] nvmem: core: Fix OOB read for bit offsets of more than one
 byte

When the bit offset is BITS_PER_BYTE or larger the read position is
advanced by `bytes_offset`. This is not taken into account in the
per-byte read loop which still reads `cell->bytes` resulting in an out of
bounds read of `bytes_offset` bytes. The information read OOB does not
leak directly as the erroneously read bits are cleared.

Detected by KASAN while looking for a use-after-free in simplefb.c.

Fixes: 7a06ef7510779 ("nvmem: core: fix bit offsets of more than one byte")
Signed-off-by: Janne Grunau <j@...nau.net>
---
 drivers/nvmem/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 387c88c55259541446901f0e539bbb0dd8c4c3de..19be16943ee66e845860192b8f008539873f6f7f 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1618,12 +1618,14 @@ static void nvmem_shift_read_buffer_in_place(struct nvmem_cell_entry *cell, void
 		*p = *b++ >> bit_offset;
 
 		/* setup rest of the bytes if any */
-		for (i = 1; i < cell->bytes; i++) {
+		for (i = 1; i < (cell->bytes - bytes_offset); i++) {
 			/* Get bits from next byte and shift them towards msb */
 			*p++ |= *b << (BITS_PER_BYTE - bit_offset);
 
 			*p = *b++ >> bit_offset;
 		}
+		/* point to end of the buffer unused bits will be cleared */
+		p = buf + cell->bytes - 1;
 	} else if (p != b) {
 		memmove(p, b, cell->bytes - bytes_offset);
 		p += cell->bytes - 1;

---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250901-nvmem-read-oob-bit-offset-dc1c2f39af6c

Best regards,
-- 
Janne Grunau <j@...nau.net>