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From:   Sasha Levin <sashal@kernel.org>
To:     tglx@linutronix.de, luto@kernel.org, ak@linux.intel.com
Cc:     corbet@lwn.net, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
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        peterz@infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, jarkko.sakkinen@linux.intel.com,
        "H . Peter Anvin" <hpa@zytor.com>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH v13 01/16] x86/ptrace: Prevent ptrace from clearing the FS/GS selector
Date:   Thu, 28 May 2020 16:13:47 -0400
Message-Id: <20200528201402.1708239-2-sashal@kernel.org>
In-Reply-To: <20200528201402.1708239-1-sashal@kernel.org>
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From: "Chang S. Bae" <chang.seok.bae@intel.com>

When a ptracer writes a ptracee's FS/GSBASE with a different value, the
selector is also cleared. This behavior is not correct as the selector
should be preserved.

Update only the base value and leave the selector intact. To simplify the
code further remove the conditional checking for the same value as this
code is not performance critical.

The only recognizable downside of this change is when the selector is
already nonzero on write. The base will be reloaded according to the
selector. But the case is highly unexpected in real usages.

[ tglx: Massage changelog ]

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/9040CFCD-74BD-4C17-9A01-B9B713CF6B10@intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kernel/ptrace.c | 17 ++---------------
 1 file changed, 2 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index f0e1ddbc2fd7..cc56efb75d27 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -380,25 +380,12 @@ static int putreg(struct task_struct *child,
 	case offsetof(struct user_regs_struct,fs_base):
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
-		/*
-		 * When changing the FS base, use do_arch_prctl_64()
-		 * to set the index to zero and to set the base
-		 * as requested.
-		 *
-		 * NB: This behavior is nonsensical and likely needs to
-		 * change when FSGSBASE support is added.
-		 */
-		if (child->thread.fsbase != value)
-			return do_arch_prctl_64(child, ARCH_SET_FS, value);
+		x86_fsbase_write_task(child, value);
 		return 0;
 	case offsetof(struct user_regs_struct,gs_base):
-		/*
-		 * Exactly the same here as the %fs handling above.
-		 */
 		if (value >= TASK_SIZE_MAX)
 			return -EIO;
-		if (child->thread.gsbase != value)
-			return do_arch_prctl_64(child, ARCH_SET_GS, value);
+		x86_gsbase_write_task(child, value);
 		return 0;
 #endif
 	}
-- 
2.25.1