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From:   Rafael Aquini <aquini@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, mcgrof@kernel.org,
        keescook@chromium.org, yzaikin@google.com
Subject: [PATCH] kernel: sysctl: ignore invalid taint bits introduced via kernel.tainted and taint the kernel with TAINT_USER on writes
Date:   Mon, 11 May 2020 17:59:04 -0400
Message-Id: <20200511215904.719257-1-aquini@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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The sysctl knob allows any user with SYS_ADMIN capability to
taint the kernel with any arbitrary value, but this might
produce an invalid flags bitset being committed to tainted_mask.

This patch introduces a simple way for proc_taint() to ignore
any eventual invalid bit coming from the user input before
committing those bits to the kernel tainted_mask, as well as
it makes clear use of TAINT_USER flag to mark the kernel
tainted by user everytime a taint value is written
to the kernel.tainted sysctl.

Signed-off-by: Rafael Aquini <aquini@redhat.com>
---
 kernel/sysctl.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 8a176d8727a3..f0a4fb38ac62 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2623,17 +2623,32 @@ static int proc_taint(struct ctl_table *table, int write,
 		return err;
 
 	if (write) {
+		int i;
+
+		/*
+		 * Ignore user input that would make us committing
+		 * arbitrary invalid TAINT flags in the loop below.
+		 */
+		tmptaint &= (1UL << TAINT_FLAGS_COUNT) - 1;
+
 		/*
 		 * Poor man's atomic or. Not worth adding a primitive
 		 * to everyone's atomic.h for this
 		 */
-		int i;
 		for (i = 0; i < BITS_PER_LONG && tmptaint >> i; i++) {
 			if ((tmptaint >> i) & 1)
 				add_taint(i, LOCKDEP_STILL_OK);
 		}
+
+		/*
+		 * Users with SYS_ADMIN capability can include any arbitrary
+		 * taint flag by writing to this interface. If that's the case,
+		 * we also need to mark the kernel "tainted by user".
+		 */
+		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
 	}
 
+
 	return err;
 }
 
-- 
2.25.4