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

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

[Janne Grunau]

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@jannau.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@jannau.net>

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

[Greg Kroah-Hartman]

On Mon, Sep 01, 2025 at 09:29:43AM +0200, Janne Grunau wrote:
> 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@jannau.net>
> ---
>  drivers/nvmem/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
as indicated below:

- You have marked a patch with a "Fixes:" tag for a commit that is in an
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  applied to any older kernel releases.  To properly fix this, please
  follow the documented rules in the
  Documentation/process/stable-kernel-rules.rst file for how to resolve
  this.

If you wish to discuss this problem further, or you have questions about
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Greg will reply once he has dug out from the pending patches received
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thanks,

greg k-h's patch email bot