linux-kernel - [PATCH 1/1] block: fix conversion of GPT partition name to 7-bit

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Message-ID: <892c7de5-d257-d4d6-2bfc-62f543965cff@canonical.com>
Date:   Tue, 23 May 2023 00:49:10 +0200
From:   Olivier Gayot <olivier.gayot@...onical.com>
To:     linux-kernel@...r.kernel.org
Cc:     Olivier Gayot <olivier.gayot@...onical.com>,
        Daniel Bungert <daniel.bungert@...onical.com>
Subject: [PATCH 1/1] block: fix conversion of GPT partition name to 7-bit

The utf16_le_to_7bit function claims to, naively, convert a UTF-16
string to a 7-bit ASCII string. By naively, we mean that it:
 * drops the first byte of every character in the original UTF-16 string
 * checks if all characters are printable, and otherwise replaces them
   by exclamation mark "!".

This means that theoretically, all characters outside the 7-bit ASCII
range should be replaced by another character. Examples:

 * lower-case alpha (ɒ) 0x0252 becomes 0x52 (R)
 * ligature OE (œ) 0x0153 becomes 0x53 (S)
 * hangul letter pieup (ㅂ) 0x3142 becomes 0x42 (B)
 * upper-case gamma (Ɣ) 0x0194 becomes 0x94 (not printable) so gets
   replaced by "!"

The result of this conversion for the GPT partition name is passed to
user-space as PARTNAME via udev, which is confusing and feels questionable.

However, there is a flaw in the conversion function itself. By dropping
one byte of each character and using isprint() to check if the remaining
byte corresponds to a printable character, we do not actually guarantee
that the resulting character is 7-bit ASCII.

This happens because we pass 8-bit characters to isprint(), which
in the kernel returns 1 for many values > 0x7f - as defined in ctype.c.

This results in many values which should be replaced by "!" to be kept
as-is, despite not being valid 7-bit ASCII. Examples:

 * e with acute accent (é) 0x00E9 becomes 0xE9 - kept as-is because
   isprint(0xE9) returns 1.
 * euro sign (€) 0x20AC becomes 0xAC - kept as-is because isprint(0xAC)
   returns 1.

Fixed by using a mask of 7 bits instead of 8 bits before calling
isprint.

Signed-off-by: Olivier Gayot <olivier.gayot@...onical.com>
---
 block/partitions/efi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 5e9be13a56a8..7acba66eed48 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -682,7 +682,7 @@ static void utf16_le_to_7bit(const __le16 *in, unsigned int size, u8 *out)
 	out[size] = 0;

 	while (i < size) {
-		u8 c = le16_to_cpu(in[i]) & 0xff;
+		u8 c = le16_to_cpu(in[i]) & 0x7f;

 		if (c && !isprint(c))
 			c = '!';