linux-hardening - [PATCH 1/3] efi: pstore: Request at most 512 bytes for variable names

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Date: Fri, 15 Mar 2024 01:25:58 +0100
From: Tim Schumacher <timschumi@....de>
To: linux-efi@...r.kernel.org
Cc: Tim Schumacher <timschumi@....de>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>,
	"Guilherme G. Piccoli" <gpiccoli@...lia.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	linux-hardening@...r.kernel.org
Subject: [PATCH 1/3] efi: pstore: Request at most 512 bytes for variable names

Work around a quirk in a few old (2011-ish) UEFI implementations, where
a call to `GetNextVariableName` with a buffer size larger than 512 bytes
will always return EFI_INVALID_PARAMETER.

This was already done to efivarfs in f45812cc23fb ("efivarfs: Request at
most 512 bytes for variable names"), but the second copy of the variable
iteration implementation was overlooked.

Signed-off-by: Tim Schumacher <timschumi@....de>
---
I CC'd the pstore people and linux-hardening mailing list because
get_maintainer.pl suggested to do so. Apologies in case this was the
incorrect decision, this is a very non-pstore-specific patch after all.

I have taken the liberty of adding a TODO for the future, the actual
refactor can follow at some point down the line.
---
 drivers/firmware/efi/efi-pstore.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/efi-pstore.c b/drivers/firmware/efi/efi-pstore.c
index e7b9ec6f8a86..f0ceb5702d21 100644
--- a/drivers/firmware/efi/efi-pstore.c
+++ b/drivers/firmware/efi/efi-pstore.c
@@ -135,7 +135,15 @@ static ssize_t efi_pstore_read(struct pstore_record *record)
 	efi_status_t status;

 	for (;;) {
-		varname_size = 1024;
+		/*
+		 * A small set of old UEFI implementations reject sizes
+		 * above a certain threshold, the lowest seen in the wild
+		 * is 512.
+		 *
+		 * TODO: Commonize with the iteration implementation in
+		 *       fs/efivarfs to keep all the quirks in one place.
+		 */
+		varname_size = 512;

 		/*
 		 * If this is the first read() call in the pstore enumeration,
--
2.44.0