Currently, the verifier doesn't reject unaligned access for map_value_adj
register types. Commit 484611357c ("bpf: allow access into map value
arrays") added logic to check_ptr_alignment() extending it from PTR_TO_PACKET
to also PTR_TO_MAP_VALUE_ADJ, but for PTR_TO_MAP_VALUE_ADJ no enforcement
is in place, because reg->id for PTR_TO_MAP_VALUE_ADJ reg types is never
non-zero, meaning, we can cause BPF_H/_W/_DW-based unaligned access for
architectures not supporting efficient unaligned access, and thus worst
case could raise exceptions on some archs that are unable to correct the
unaligned access or perform a different memory access to the actual
requested one and such.
i) Unaligned load with !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
on r0 (map_value_adj):
0: (bf) r2 = r10
1: (07) r2 += -8
2: (7a) *(u64 *)(r2 +0) = 0
3: (18) r1 = 0x42533a00
5: (85) call bpf_map_lookup_elem#1
6: (15) if r0 == 0x0 goto pc+11
R0=map_value(ks=8,vs=48,id=0),min_value=0,max_value=0 R10=fp
7: (61) r1 = *(u32 *)(r0 +0)
8: (35) if r1 >= 0xb goto pc+9
R0=map_value(ks=8,vs=48,id=0),min_value=0,max_value=0 R1=inv,min_value=0,max_value=10 R10=fp
9: (07) r0 += 3
10: (79) r7 = *(u64 *)(r0 +0)
R0=map_value_adj(ks=8,vs=48,id=0),min_value=3,max_value=3 R1=inv,min_value=0,max_value=10 R10=fp
11: (79) r7 = *(u64 *)(r0 +2)
R0=map_value_adj(ks=8,vs=48,id=0),min_value=3,max_value=3 R1=inv,min_value=0,max_value=10 R7=inv R10=fp
[...]
ii) Unaligned store with !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
on r0 (map_value_adj):
0: (bf) r2 = r10
1: (07) r2 += -8
2: (7a) *(u64 *)(r2 +0) = 0
3: (18) r1 = 0x4df16a00
5: (85) call bpf_map_lookup_elem#1
6: (15) if r0 == 0x0 goto pc+19
R0=map_value(ks=8,vs=48,id=0),min_value=0,max_value=0 R10=fp
7: (07) r0 += 3
8: (7a) *(u64 *)(r0 +0) = 42
R0=map_value_adj(ks=8,vs=48,id=0),min_value=3,max_value=3 R10=fp
9: (7a) *(u64 *)(r0 +2) = 43
R0=map_value_adj(ks=8,vs=48,id=0),min_value=3,max_value=3 R10=fp
10: (7a) *(u64 *)(r0 -2) = 44
R0=map_value_adj(ks=8,vs=48,id=0),min_value=3,max_value=3 R10=fp
[...]
For the PTR_TO_PACKET type, reg->id is initially zero when skb->data
was fetched, it later receives a reg->id from env->id_gen generator
once another register with UNKNOWN_VALUE type was added to it via
check_packet_ptr_add(). The purpose of this reg->id is twofold: i) it
is used in find_good_pkt_pointers() for setting the allowed access
range for regs with PTR_TO_PACKET of same id once verifier matched
on data/data_end tests, and ii) for check_ptr_alignment() to determine
that when not having efficient unaligned access and register with
UNKNOWN_VALUE was added to PTR_TO_PACKET, that we're only allowed
to access the content bytewise due to unknown unalignment. reg->id
was never intended for PTR_TO_MAP_VALUE{,_ADJ} types and thus is
always zero, the only marking is in PTR_TO_MAP_VALUE_OR_NULL that
was added after 484611357c via 57a09bf0a4 ("bpf: Detect identical
PTR_TO_MAP_VALUE_OR_NULL registers"). Above tests will fail for
non-root environment due to prohibited pointer arithmetic.
The fix splits register-type specific checks into their own helper
instead of keeping them combined, so we don't run into a similar
issue in future once we extend check_ptr_alignment() further and
forget to add reg->type checks for some of the checks.
Fixes: 484611357c ("bpf: allow access into map value arrays")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
This file was moved to Documentation/admin-guide/README.rst
Please notice that there are several guides for kernel developers and users.
These guides can be rendered in a number of formats, like HTML and PDF.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.
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.
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