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    SubjectRE: [RESEND PATCH v1] proc: use untagged_addr() for pagemap_read addresses
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    > -----Original Message-----
    > From: Miles Chen [mailto:miles.chen@mediatek.com]
    > Sent: Monday, November 23, 2020 7:39 PM
    > To: Alexey Dobriyan <adobriyan@gmail.com>; Andrew Morton
    > <akpm@linux-foundation.org>
    > Cc: linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org;
    > linux-mediatek@lists.infradead.org; wsd_upstream@mediatek.com; Miles
    > Chen <miles.chen@mediatek.com>
    > Subject: [RESEND PATCH v1] proc: use untagged_addr() for pagemap_read
    > addresses
    >
    > 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
    >
    > === 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);
    > 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;

    Wouldn't the untag be done by the user reading pagemap file?
    With this patch, even users pass an illegal address, for example,
    users put a tag on a virtual address which hasn't really a tag,
    they will still get the right pagemap.

    >
    > /*
    > --
    > 2.18.0

    Thanks
    Barry

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