https://github.com/torvalds/linux
Revision 2c2ade81741c66082f8211f0b96cf509cc4c0218 authored by Jann Horn on 13 February 2019, 21:45:59 UTC, committed by David S. Miller on 14 February 2019, 17:12:17 UTC
The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum number of references that we might need to create in the fastpath later, the bump-allocation fastpath only has to modify the non-atomic bias value that tracks the number of extra references we hold instead of the atomic refcount. The maximum number of allocations we can serve (under the assumption that no allocation is made with size 0) is nc->size, so that's the bias used. However, even when all memory in the allocation has been given away, a reference to the page is still held; and in the `offset < 0` slowpath, the page may be reused if everyone else has dropped their references. This means that the necessary number of references is actually `nc->size+1`. Luckily, from a quick grep, it looks like the only path that can call page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which requires CAP_NET_ADMIN in the init namespace and is only intended to be used for kernel testing and fuzzing. To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the `offset < 0` path, below the virt_to_page() call, and then repeatedly call writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI, with a vector consisting of 15 elements containing 1 byte each. Signed-off-by: Jann Horn <jannh@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
1 parent 61c4c0b
Tip revision: 2c2ade81741c66082f8211f0b96cf509cc4c0218 authored by Jann Horn on 13 February 2019, 21:45:59 UTC
mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs
mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs
Tip revision: 2c2ade8
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arch | ||
block | ||
certs | ||
crypto | ||
drivers | ||
firmware | ||
fs | ||
include | ||
init | ||
ipc | ||
kernel | ||
lib | ||
mm | ||
net | ||
samples | ||
scripts | ||
security | ||
sound | ||
tools | ||
usr | ||
virt | ||
.clang-format | -rw-r--r-- | 13.9 KB |
.cocciconfig | -rw-r--r-- | 59 bytes |
.get_maintainer.ignore | -rw-r--r-- | 31 bytes |
.gitattributes | -rw-r--r-- | 30 bytes |
.gitignore | -rw-r--r-- | 1.5 KB |
.mailmap | -rw-r--r-- | 10.7 KB |
COPYING | -rw-r--r-- | 423 bytes |
CREDITS | -rw-r--r-- | 96.9 KB |
Kbuild | -rw-r--r-- | 1.7 KB |
Kconfig | -rw-r--r-- | 563 bytes |
MAINTAINERS | -rw-r--r-- | 482.2 KB |
Makefile | -rw-r--r-- | 59.1 KB |
README | -rw-r--r-- | 727 bytes |
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