linux-kernel - [PATCH 04/12] futex: Validate futex value against futex size

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Message-Id: <20230728164235.1318118-5-axboe@kernel.dk>
Date:   Fri, 28 Jul 2023 10:42:27 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, andres@...razel.de, tglx@...utronix.de,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 04/12] futex: Validate futex value against futex size

From: Peter Zijlstra <peterz@...radead.org>

Ensure the futex value fits in the given futex size. Since this adds a
constraint to an existing syscall, it might possibly change behaviour.

Currently the value would be truncated to a u32 and any high bits
would get silently lost.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
---
 kernel/futex/futex.h    | 8 ++++++++
 kernel/futex/syscalls.c | 3 +++
 2 files changed, 11 insertions(+)

diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index c0e04599904a..d0a43d751e30 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -86,6 +86,14 @@ static inline unsigned int futex_size(unsigned int flags)
 	return 1 << size; /* {0,1,2,3} -> {1,2,4,8} */
 }
 
+static inline bool futex_validate_input(unsigned int flags, u64 val)
+{
+	int bits = 8 * futex_size(flags);
+	if (bits < 64 && (val >> bits))
+		return false;
+	return true;
+}
+
 #ifdef CONFIG_FAIL_FUTEX
 extern bool should_fail_futex(bool fshared);
 #else
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index 36824b64219a..d2b2bcf2a665 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -209,6 +209,9 @@ static int futex_parse_waitv(struct futex_vector *futexv,
 		if (!futex_flags_valid(flags))
 			return -EINVAL;
 
+		if (!futex_validate_input(flags, aux.val))
+			return -EINVAL;
+
 		futexv[i].w.flags = flags;
 		futexv[i].w.val = aux.val;
 		futexv[i].w.uaddr = aux.uaddr;
-- 
2.40.1