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Subject: Re: [patch V2 1/3] x86/ldt: Plug memory leak in error path
From: Andy Lutomirski <luto@amacapital.net>
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In-Reply-To: <alpine.DEB.2.20.1712311121340.1899@nanos>
Date: Sun, 31 Dec 2017 07:23:18 -0800
Cc: LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Message-Id: <72F8C4C3-B83A-4F4E-A900-644EC359E383@amacapital.net>
References: <20171230211351.980176980@linutronix.de> <20171230211829.508293470@linutronix.de> <alpine.DEB.2.20.1712311121340.1899@nanos>
To: Thomas Gleixner <tglx@linutronix.de>
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> On Dec 31, 2017, at 2:24 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> The error path in write_ldt() tries to free old_ldt instead of the newly
> allocated new_ldt resulting in a memory leak. It also misses to clean up a
> half populated LDT pagetable, which is not a leak as it gets cleaned up
> when the process exits.
> 
> Free both the potentially half populated LDT pagetable and the newly
> allocated LDT struct. This can be done unconditionally because once a LDT
> is mapped subsequent maps will succeed because the PTE page is already
> populated and the two LDTs fit into that single page.
> 
> Fixes: f55f0501cbf6 ("x86/pti: Put the LDT in its own PGD if PTI is on")
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/x86/kernel/ldt.c |    8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/kernel/ldt.c
> +++ b/arch/x86/kernel/ldt.c
> @@ -421,7 +421,13 @@ static int write_ldt(void __user *ptr, u
>     */
>    error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
>    if (error) {
> -        free_ldt_struct(old_ldt);
> +        /*
> +         * This only can fail for the first LDT setup. If a LDT is
> +         * already installed then the PTE page is already
> +         * populated. Mop up a half populated page table.
> +         */

I liked it better with the conditional.  If this ever fails due to fault injection, some silly accounting issue, a paravirt glitch, or whatever, then we'll oops.

> +        free_ldt_pgtables(mm);
> +        free_ldt_struct(new_ldt);
>        goto out_unlock;
>    }
>