linux/tools
Gianluca Borello 06c1c04972 bpf: allow helpers access to variable memory
Currently, helpers that read and write from/to the stack can do so using
a pair of arguments of type ARG_PTR_TO_STACK and ARG_CONST_STACK_SIZE.
ARG_CONST_STACK_SIZE accepts a constant register of type CONST_IMM, so
that the verifier can safely check the memory access. However, requiring
the argument to be a constant can be limiting in some circumstances.

Since the current logic keeps track of the minimum and maximum value of
a register throughout the simulated execution, ARG_CONST_STACK_SIZE can
be changed to also accept an UNKNOWN_VALUE register in case its
boundaries have been set and the range doesn't cause invalid memory
accesses.

One common situation when this is useful:

int len;
char buf[BUFSIZE]; /* BUFSIZE is 128 */

if (some_condition)
	len = 42;
else
	len = 84;

some_helper(..., buf, len & (BUFSIZE - 1));

The compiler can often decide to assign the constant values 42 or 48
into a variable on the stack, instead of keeping it in a register. When
the variable is then read back from stack into the register in order to
be passed to the helper, the verifier will not be able to recognize the
register as constant (the verifier is not currently tracking all
constant writes into memory), and the program won't be valid.

However, by allowing the helper to accept an UNKNOWN_VALUE register,
this program will work because the bitwise AND operation will set the
range of possible values for the UNKNOWN_VALUE register to [0, BUFSIZE),
so the verifier can guarantee the helper call will be safe (assuming the
argument is of type ARG_CONST_STACK_SIZE_OR_ZERO, otherwise one more
check against 0 would be needed). Custom ranges can be set not only with
ALU operations, but also by explicitly comparing the UNKNOWN_VALUE
register with constants.

Another very common example happens when intercepting system call
arguments and accessing user-provided data of variable size using
bpf_probe_read(). One can load at runtime the user-provided length in an
UNKNOWN_VALUE register, and then read that exact amount of data up to a
compile-time determined limit in order to fit into the proper local
storage allocated on the stack, without having to guess a suboptimal
access size at compile time.

Also, in case the helpers accepting the UNKNOWN_VALUE register operate
in raw mode, disable the raw mode so that the program is required to
initialize all memory, since there is no guarantee the helper will fill
it completely, leaving possibilities for data leak (just relevant when
the memory used by the helper is the stack, not when using a pointer to
map element value or packet). In other words, ARG_PTR_TO_RAW_STACK will
be treated as ARG_PTR_TO_STACK.

Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
2017-01-09 16:56:27 -05:00
..
accounting tools: move accounting tool from Documentation 2016-09-23 13:07:15 -06:00
arch Merge branch 'core/urgent' into x86/fpu, to merge fixes 2016-11-01 07:47:40 +01:00
build Merge branch 'kbuild' of git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild 2016-12-17 16:24:13 -08:00
cgroup
firewire
gpio Merge branch 'kbuild' of git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild 2016-12-17 16:24:13 -08:00
hv Char/Misc driver patches for 4.10-rc1 2016-12-13 12:11:01 -08:00
iio tools: iio: iio_generic_buffer: drop unneeded parentheses 2016-10-23 19:34:10 +01:00
include Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-23 16:49:12 -08:00
kvm/kvm_stat
laptop tools: move laptops dslm tool from Documentation 2016-09-23 13:07:21 -06:00
leds tools/leds: Add uledmon program for monitoring userspace LEDs 2016-11-22 12:07:02 +01:00
lguest tools/lguest: Don't bork the terminal in case of wrong args 2016-09-08 08:05:24 +02:00
lib Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-23 16:49:12 -08:00
net
nfsd
objtool Merge branch 'kbuild' of git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild 2016-12-17 16:24:13 -08:00
pcmcia tools: move pcmcia crc32hash tool from Documentation 2016-09-23 13:07:27 -06:00
perf Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-23 16:49:12 -08:00
power Merge branch 'turbostat' of git://git.kernel.org/pub/scm/linux/kernel/git/lenb/linux 2016-12-25 14:01:28 -08:00
scripts tools build: Fix objtool build with ARCH=x86_64 2016-07-22 16:37:44 -03:00
spi spi: spidev_test: Fix input file check when transferring file 2016-11-04 09:56:09 -06:00
testing bpf: allow helpers access to variable memory 2017-01-09 16:56:27 -05:00
thermal/tmon
time
usb usbip: add missing compile time generated files to .gitignore 2016-12-06 08:37:41 +01:00
virtio tools/virtio: use {READ,WRITE}_ONCE() in uaccess.h 2016-12-16 00:13:37 +02:00
vm tools/vm/page_owner: increase temporary buffer size 2016-07-26 16:19:19 -07:00
Makefile tools/leds: Add uledmon program for monitoring userspace LEDs 2016-11-22 12:07:02 +01:00