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[PATCH v3] efidisk: pass buffers with higher alignment

From: Stefan Agner
Subject: [PATCH v3] efidisk: pass buffers with higher alignment
Date: Wed, 18 May 2022 10:59:05 +0200

Some devices report a IoAlign value of 2, however seem to require a
buffer with higher alignment.

The UEFI specification is saying: "IoAlign values of 0 and 1 mean that
the buffer can be placed anywhere in memory. Otherwise, IoAlign must
be a power of 2, and the requirement is that the start address of a
buffer must be evenly divisible by IoAlign with no remainder."

It seems that this got misinterpreted by some vendors assuming IoAlign
is 2^IoAlign. There is also such a hint in an example in earlier
versions of the Driver Writer's Guide:
ScsiPassThruMode.IoAlign = 2; // Data must be alligned on 4-byte boundary

However, it is unsafe to just blindly align buffers by 2^IoAlign, as
this would lead to an overflow for systems which use block size
alignment (e.g. 512 bytes, for example U-Boot).

Work around by using an alignment of at least BlockSize (typically 512
bytes) in case alignment is requested. Also make sure that IoAlign is
still respected as per UEFI specification if a higher alignment than
block size is requested.

Note: The problem has only noticed with compressed squashfs. It seems
that ext4 (and presumably other file system drivers) pass buffers with
a higher alignment already.

Acked-by: Heinrich Schuchardt <>
Signed-off-by: Stefan Agner <>
 grub-core/disk/efi/efidisk.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/grub-core/disk/efi/efidisk.c b/grub-core/disk/efi/efidisk.c
index 9e20af70e..c4eb4f4e7 100644
--- a/grub-core/disk/efi/efidisk.c
+++ b/grub-core/disk/efi/efidisk.c
@@ -553,8 +553,22 @@ grub_efidisk_readwrite (struct grub_disk *disk, 
grub_disk_addr_t sector,
   d = disk->data;
   bio = d->block_io;
-  /* Set alignment to 1 if 0 specified */
-  io_align = bio->media->io_align ? bio->media->io_align : 1;
+  /*
+   * If IoAlign is > 1, it should represent the required alignment. However,
+   * some UEFI implementation on Intel NUC systems seem to use IoAlign=2 but
+   * require 2^IoAlign.
+   * Make sure to align to at least block size if IO alignment is required.
+   */
+  if (bio->media->io_align > 1)
+    {
+      if (bio->media->io_align < bio->media->block_size)
+        io_align = bio->media->block_size;
+      else
+        io_align = bio->media->io_align;
+    }
+  else
+    io_align = 1;
   num_bytes = size << disk->log_sector_size;
   if ((grub_addr_t) buf & (io_align - 1))

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