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Greg Hackmann
5ad356eabc
ARM64's pfn_valid() shifts away the upper PAGE_SHIFT bits of the input
before seeing if the PFN is valid. This leads to false positives when
some of the upper bits are set, but the lower bits match a valid PFN.
For example, the following userspace code looks up a bogus entry in
/proc/kpageflags:
int pagemap = open("/proc/self/pagemap", O_RDONLY);
int pageflags = open("/proc/kpageflags", O_RDONLY);
uint64_t pfn, val;
lseek64(pagemap, [...], SEEK_SET);
read(pagemap, &pfn, sizeof(pfn));
if (pfn & (1UL << 63)) { /* valid PFN */
pfn &= ((1UL << 55) - 1); /* clear flag bits */
pfn |= (1UL << 55);
lseek64(pageflags, pfn * sizeof(uint64_t), SEEK_SET);
read(pageflags, &val, sizeof(val));
}
On ARM64 this causes the userspace process to crash with SIGSEGV rather
than reading (1 << KPF_NOPAGE). kpageflags_read() treats the offset as
valid, and stable_page_flags() will try to access an address between the
user and kernel address ranges.
Fixes: c1cc155261 ("arm64: MMU initialisation")
Cc: stable@vger.kernel.org
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
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In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.