+ proc-use-untagged_addr-for-pagemap_read-addresses.patch added to -mm tree - Linux-stable-mirror

28 Nov 2020

The patch titled
     Subject: proc: use untagged_addr() for pagemap_read addresses
has been added to the -mm tree.  Its filename is
     proc-use-untagged_addr-for-pagemap_read-addresses.patch
This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/proc-use-untagged_addr-for-pagemap...
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/proc-use-untagged_addr-for-pagemap...
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------------------------------------------------------
From: Miles Chen miles.chen@mediatek.com
Subject: 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.
Explanation from Catalin in [1]:
: Arguably, that's a user-space bug since tagged file offsets were never
: supported.  In this case it's not even a tag at bit 56 as per the arm64
: tagged address ABI but rather down to bit 47.  You could say that the
: problem is caused by the C library (malloc()) or whoever created the
: tagged vaddr and passed it to this function.  It's not a kernel regression
: as we've never supported it.
: 
: Now, pagemap is a special case where the offset is usually not generated
: as a classic file offset but rather derived by shifting a user virtual
: address.  I guess we can make a concession for pagemap (only) and allow
: such offset with the tag at bit (56 - PAGE_SHIFT + 3).
My test code is baed on [2]:
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://lore.kernel.org/patchwork/patch/1343258/
[2] https://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158
Link: https://lkml.kernel.org/r/20201127050738.14440-1-miles.chen@mediatek.com
Signed-off-by: Miles Chen miles.chen@mediatek.com
Cc: Alexey Dobriyan adobriyan@gmail.com
Cc: Andrey Konovalov andreyknvl@google.com
Cc: Alexander Potapenko glider@google.com
Cc: Vincenzo Frascino vincenzo.frascino@arm.com
Cc: Andrey Ryabinin aryabinin@virtuozzo.com
Cc: Catalin Marinas catalin.marinas@arm.com
Cc: Dmitry Vyukov dvyukov@google.com
Cc: Marco Elver elver@google.com
Cc: Will Deacon will.deacon@arm.com
Cc: Eric W. Biederman ebiederm@xmission.com
Cc: Song Bao Hua (Barry Song) song.bao.hua@hisilicon.com
Cc: stable@vger.kernel.org	[5.4+]
Signed-off-by: Andrew Morton akpm@linux-foundation.org
---
fs/proc/task_mmu.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/fs/proc/task_mmu.c~proc-use-untagged_addr-for-pagemap_read-addresses
+++ a/fs/proc/task_mmu.c
@@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file
src = *ppos;
    svpfn = src / PM_ENTRY_BYTES;
-	start_vaddr = svpfn << PAGE_SHIFT;
    end_vaddr = mm->task_size;
/* watch out for wraparound */
-	if (svpfn > mm->task_size >> PAGE_SHIFT)
+	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;
/*
_
Patches currently in -mm which might be from miles.chen@mediatek.com are
proc-use-untagged_addr-for-pagemap_read-addresses.patch

    