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[PATCH v3 6/8] efi/efinet: Don't close connections at fini_hw() time

From: Ard Biesheuvel
Subject: [PATCH v3 6/8] efi/efinet: Don't close connections at fini_hw() time
Date: Thu, 18 Aug 2022 16:51:33 +0200

When GRUB runs on top of EFI firmware, it only has access to block and
network device abstractions exposed by the firmware, and it is up to the
firmware to quiesce the underlying hardware when handing over to the OS.

This is especially important for network devices, to prevent incoming
packets from being DMA'd straight into memory after the OS has taken
over but before it has managed to reconfigure the network hardware.

GRUB handles this by means of the grub_net_fini_hw() preboot hook, which
is executed before calling into the booted image. This means that all
network devices disappear or become inoperable before the EFI stub
executes on EFI targeted builds. This is problematic as it prevents the
EFI stub from calling back into GRUB provided protocols such as
LoadFile2 for the initrd, which we will provide in a subsequent patch.

So add a flag that indicates to the network core that EFI network
devices should not be closed when grub_net_fini_hw() is called.

Signed-off-by: Ard Biesheuvel <>
 grub-core/net/drivers/efi/efinet.c | 10 +++++++++-
 grub-core/net/net.c                |  2 +-
 include/grub/net.h                 |  3 ++-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/grub-core/net/drivers/efi/efinet.c 
index 73343d26d9e1..5adf5f40f492 100644
--- a/grub-core/net/drivers/efi/efinet.c
+++ b/grub-core/net/drivers/efi/efinet.c
@@ -320,7 +320,15 @@ grub_efinet_findcards (void)
       card->name = grub_xasprintf ("efinet%d", i++);
       card->driver = &efidriver;
-      card->flags = 0;
+      /*
+       * EFI network devices are abstract SNP protocol instances, and the
+       * firmware is in charge of ensuring that they will be torn down when the
+       * OS loader hands off to the OS proper. Closing them as part of the
+       * preboot cleanup is therefore unnecessary, and undesirable, as it
+       * prevents us from using the network connection in a protocal callback
+       * such as LoadFile2 for initrd loading.
+       */
+      card->flags = GRUB_NET_CARD_NO_CLOSE_ON_FINI_HW;
       card->default_address.type = GRUB_NET_LINK_LEVEL_PROTOCOL_ETHERNET;
       grub_memcpy (card->default_address.mac,
diff --git a/grub-core/net/net.c b/grub-core/net/net.c
index 064e7114e012..7046dc57890a 100644
--- a/grub-core/net/net.c
+++ b/grub-core/net/net.c
@@ -1787,7 +1787,7 @@ grub_net_fini_hw (int noreturn __attribute__ ((unused)))
   struct grub_net_card *card;
   FOR_NET_CARDS (card)
-    if (card->opened)
+    if (card->opened && !(card->flags & GRUB_NET_CARD_NO_CLOSE_ON_FINI_HW))
        if (card->driver->close)
          card->driver->close (card);
diff --git a/include/grub/net.h b/include/grub/net.h
index a64a04cc80b1..79cba357ae6a 100644
--- a/include/grub/net.h
+++ b/include/grub/net.h
@@ -64,7 +64,8 @@ typedef enum grub_net_interface_flags
 typedef enum grub_net_card_flags
   } grub_net_card_flags_t;
 struct grub_net_card;

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