linux-kernel - [PATCH v3] efivarfs: Request at most 512 bytes for variable names

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Date: Fri, 26 Jan 2024 17:25:23 +0100
From: Tim Schumacher <timschumi@....de>
To: linux-efi@...r.kernel.org
Cc: Tim Schumacher <timschumi@....de>,
	jk@...abs.org,
	ardb@...nel.org,
	mjg59@...f.ucam.org,
	pjones@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3] efivarfs: Request at most 512 bytes for variable names

This sidesteps 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`.

Cc: stable@...r.kernel.org # 6.1+
Signed-off-by: Tim Schumacher <timschumi@....de>
---
Made a patch with just the size change (and a comment / error message
for good measure) as requested. I just hope that I don't have to come
back in a month to find out that there is a machine that only accepts up
to (e.g.) 256 bytes.

One thing that I just recently noticed is that properly processing
variables above 512 bytes in size is currently meaningless anyways,
since the VFS layer only allows file name sizes of up to 255 bytes,
and 512 bytes of UCS2 will end up being at least 256 bytes of
UTF-8.

If path creation errors are decoupled from the iteration process then
one could at least skip those entries, but the efivarfs implementation
seems to be in no shape to handle that currently.
---
Changes since v1 ("efivarfs: Iterate variables with increasing name buffer sizes"):

- Redid the logic to start with the current limit of 1024 and continuously
  halve it until we get a successful result.
- Added a warning line in case we find anything that is bigger than the limit.
- Removed an outdated (or never accurate?) comment about a specification-imposed
  length limit.

Changes since v2 ("efivarfs: Halve name buffer size until first successful response"):

- Removed all the complicated logic, only keeping the new limit, the
  comment change, and the error message in case a big variable is found.
---
 fs/efivarfs/vars.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/efivarfs/vars.c b/fs/efivarfs/vars.c
index 9e4f47808bd5..3f6385f2a4e5 100644
--- a/fs/efivarfs/vars.c
+++ b/fs/efivarfs/vars.c
@@ -372,7 +372,7 @@ static void dup_variable_bug(efi_char16_t *str16, efi_guid_t *vendor_guid,
 int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
 		void *data, bool duplicates, struct list_head *head)
 {
-	unsigned long variable_name_size = 1024;
+	unsigned long variable_name_size = 512;
 	efi_char16_t *variable_name;
 	efi_status_t status;
 	efi_guid_t vendor_guid;
@@ -389,12 +389,13 @@ int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
 		goto free;

 	/*
-	 * Per EFI spec, the maximum storage allocated for both
-	 * the variable name and variable data is 1024 bytes.
+	 * A small set of old UEFI implementations reject sizes
+	 * above a certain threshold, the lowest seen in the wild
+	 * is 512.
 	 */

 	do {
-		variable_name_size = 1024;
+		variable_name_size = 512;

 		status = efivar_get_next_variable(&variable_name_size,
 						  variable_name,
@@ -431,6 +432,11 @@ int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
 			break;
 		case EFI_NOT_FOUND:
 			break;
+		case EFI_BUFFER_TOO_SMALL:
+			printk(KERN_WARNING "efivars: Assumed maximum name size to be 512, got name of size %lu\n",
+			       variable_name_size);
+			status = EFI_NOT_FOUND;
+			break;
 		default:
 			printk(KERN_WARNING "efivars: get_next_variable: status=%lx\n",
 				status);
--
2.43.0