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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Miles Chen <miles.chen@mediatek.com>
Cc:     Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <linux-mediatek@lists.infradead.org>, <wsd_upstream@mediatek.com>
References: <20201123063835.18981-1-miles.chen@mediatek.com>
Date:   Tue, 24 Nov 2020 12:32:56 -0600
In-Reply-To: <20201123063835.18981-1-miles.chen@mediatek.com> (Miles Chen's
        message of "Mon, 23 Nov 2020 14:38:35 +0800")
Message-ID: <87lfeqsizr.fsf@x220.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
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Subject: Re: [RESEND PATCH v1] proc: use untagged_addr() for pagemap_read addresses
Precedence: bulk

Miles Chen <miles.chen@mediatek.com> writes:

> When we try to visit the pagemap of a tagged userspace pointer, we find
> that the start_vaddr is not correct because of the tag.
> To fix it, we should untag the usespace pointers in pagemap_read().
>
> I tested with 5.10-rc4 and the issue remains.
>
> My test code is baed on [1]:
>
> A userspace pointer which has been tagged by 0xb4: 0xb400007662f541c8


Sigh this patch is buggy.

> === userspace program ===
>
> uint64 OsLayer::VirtualToPhysical(void *vaddr) {
> 	uint64 frame, paddr, pfnmask, pagemask;
> 	int pagesize = sysconf(_SC_PAGESIZE);
> 	off64_t off = ((uintptr_t)vaddr) / pagesize * 8; // off = 0xb400007662f541c8 / pagesize * 8 = 0x5a00003b317aa0
> 	int fd = open(kPagemapPath, O_RDONLY);
> 	...
>
> 	if (lseek64(fd, off, SEEK_SET) != off || read(fd, &frame, 8) != 8) {
> 		int err = errno;
> 		string errtxt = ErrorString(err);
> 		if (fd >= 0)
> 			close(fd);
> 		return 0;
> 	}
> ...
> }
>
> === kernel fs/proc/task_mmu.c ===
>
> static ssize_t pagemap_read(struct file *file, char __user *buf,
> 		size_t count, loff_t *ppos)
> {
> 	...
> 	src = *ppos;
> 	svpfn = src / PM_ENTRY_BYTES; // svpfn == 0xb400007662f54
> 	start_vaddr = svpfn << PAGE_SHIFT; // start_vaddr == 0xb400007662f54000
> 	end_vaddr = mm->task_size;
>
> 	/* watch out for wraparound */
> 	// svpfn == 0xb400007662f54
> 	// (mm->task_size >> PAGE) == 0x8000000
> 	if (svpfn > mm->task_size >> PAGE_SHIFT) // the condition is true because of the tag 0xb4
> 		start_vaddr = end_vaddr;
>
> 	ret = 0;
> 	while (count && (start_vaddr < end_vaddr)) { // we cannot visit correct entry because start_vaddr is set to end_vaddr
> 		int len;
> 		unsigned long end;
> 		...
> 	}
> 	...
> }
>
> [1] https://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158
>
> Signed-off-by: Miles Chen <miles.chen@mediatek.com>
> ---
>  fs/proc/task_mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 217aa2705d5d..e9a70f7ee515 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1599,11 +1599,11 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>  
>  	src = *ppos;
>  	svpfn = src / PM_ENTRY_BYTES;

> -	start_vaddr = svpfn << PAGE_SHIFT;
> +	start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Arguably the line above is safe, but unfortunately it has the
possibility of suffering from overflow.

>  	end_vaddr = mm->task_size;
>  
>  	/* watch out for wraparound */
> -	if (svpfn > mm->task_size >> PAGE_SHIFT)
> +	if (start_vaddr > mm->task_size)
>  		start_vaddr = end_vaddr;

Overflow handling you are removing here.
>  
>  	/*


I suspect the proper way to handle this is to move the test for
overflow earlier so the code looks something like:

	end_vaddr = mm->task_size;

	src = *ppos;
	svpfn = src / PM_ENTRY_BYTES;

	/* watch out for wraparound */
        start_vaddr = end_vaddr;
	if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
        	start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);

	/* Ensure the address is inside the task */
	if (start_vaddr > mm->task_size)
        	start_vaddr = end_vaddr;

Eric