linux-kernel - [PATCH 2/2] rseq: Kill process when unknown flags are encountered in ABI structures

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Date:   Wed, 22 Jun 2022 15:46:17 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Peter Oskolkov <posk@...k.io>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [PATCH 2/2] rseq: Kill process when unknown flags are encountered in ABI structures

rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are
currently unused.

The current behavior when those bits are set is to ignore them. This is
not an ideal behavior, because when future features will start using
those flags, if user-space fails to correctly validate that the kernel
indeed supports those flags (e.g. with a new sys_rseq flags bit) before
using them, it may incorrectly assume that the kernel will handle those
flags way when in fact those will be silently ignored on older kernels.

Validating that unused flags bits are cleared will allow a smoother
transition when those flags will start to be used by allowing
applications to fail early, and obviously, when they attempt to use the
new flags on an older kernel that does not support them.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
---
 kernel/rseq.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 81d7dc80787b..bda8175f8f99 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -176,7 +176,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	u32 flags, event_mask;
 	int ret;

-	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
+	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
 		return -EINVAL;

 	/* Get thread flags. */
@@ -184,7 +184,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
 	if (ret)
 		return ret;

-	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
+	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
 		return -EINVAL;

 	/*
-- 
2.30.2