[tip: efi/urgent] efi/libstub: Fix path separator regression

[tip-bot2 for Philipp Fent]

The following commit has been merged into the efi/urgent branch of tip:

Commit-ID:     7a88a6227dc7f2e723bba11ece05e57bd8dce8e4
Gitweb:        https://git.kernel.org/tip/7a88a6227dc7f2e723bba11ece05e57bd8dce8e4
Author:        Philipp Fent <fent@xxxxxxxxx>
AuthorDate:    Mon, 15 Jun 2020 13:51:09 +02:00
Committer:     Ard Biesheuvel <ardb@xxxxxxxxxx>
CommitterDate: Mon, 15 Jun 2020 19:43:59 +02:00

efi/libstub: Fix path separator regression

Commit 9302c1bb8e47 ("efi/libstub: Rewrite file I/O routine") introduced a
regression that made a couple of (badly configured) systems fail to
boot [1]: Until 5.6, we silently accepted Unix-style file separators in
EFI paths, which might violate the EFI standard, but are an easy to make
mistake. This fix restores the pre-5.7 behaviour.

[1] https://bbs.archlinux.org/viewtopic.php?id=256273

Fixes: 9302c1bb8e47 ("efi/libstub: Rewrite file I/O routine")
Signed-off-by: Philipp Fent <fent@xxxxxxxxx>
Link: https://lore.kernel.org/r/20200615115109.7823-1-fent@xxxxxxxxx
[ardb: rewrite as chained if/else statements]
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 drivers/firmware/efi/libstub/file.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/efi/libstub/file.c b/drivers/firmware/efi/libstub/file.c
index 2005e33..630caa6 100644
--- a/drivers/firmware/efi/libstub/file.c
+++ b/drivers/firmware/efi/libstub/file.c
@@ -102,12 +102,20 @@ static int find_file_option(const efi_char16_t *cmdline, int cmdline_len,
 	if (!found)
 		return 0;
 
+	/* Skip any leading slashes */
+	while (cmdline[i] == L'/' || cmdline[i] == L'\\')
+		i++;
+
 	while (--result_len > 0 && i < cmdline_len) {
-		if (cmdline[i] == L'\0' ||
-		    cmdline[i] == L'\n' ||
-		    cmdline[i] == L' ')
+		efi_char16_t c = cmdline[i++];
+
+		if (c == L'\0' || c == L'\n' || c == L' ')
 			break;
-		*result++ = cmdline[i++];
+		else if (c == L'/')
+			/* Replace UNIX dir separators with EFI standard ones */
+			*result++ = L'\\';
+		else
+			*result++ = c;
 	}
 	*result = L'\0';
 	return i;