[PATCH bpf] bpf: fix crash due to inode i_op mismatch with clang/llvm

[Daniel Borkmann <daniel@iogearbox.net>]

Prasad reported that he has seen crashes with netd on Android with
arm64 in the form of (note, the taint is unrelated):

  [ 4134.721483] Unable to handle kernel paging request at virtual address 800000001
  [ 4134.820925] Mem abort info:
  [ 4134.901283]   Exception class = DABT (current EL), IL = 32 bits
  [ 4135.016736]   SET = 0, FnV = 0
  [ 4135.119820]   EA = 0, S1PTW = 0
  [ 4135.201431] Data abort info:
  [ 4135.301388]   ISV = 0, ISS = 0x00000021
  [ 4135.359599]   CM = 0, WnR = 0
  [ 4135.470873] user pgtable: 4k pages, 39-bit VAs, pgd = ffffffe39b946000
  [ 4135.499757] [0000000800000001] *pgd=0000000000000000, *pud=0000000000000000
  [ 4135.660725] Internal error: Oops: 96000021 [#1] PREEMPT SMP
  [ 4135.674610] Modules linked in:
  [ 4135.682883] CPU: 5 PID: 1260 Comm: netd Tainted: G S      W       4.14.19+ #1
  [ 4135.716188] task: ffffffe39f4aa380 task.stack: ffffff801d4e0000
  [ 4135.731599] PC is at bpf_prog_add+0x20/0x68
  [ 4135.741746] LR is at bpf_prog_inc+0x20/0x2c
  [ 4135.751788] pc : [<ffffff94ab7ad584>] lr : [<ffffff94ab7ad638>] pstate: 60400145
  [ 4135.769062] sp : ffffff801d4e3ce0
  [...]
  [ 4136.258315] Process netd (pid: 1260, stack limit = 0xffffff801d4e0000)
  [ 4136.273746] Call trace:
  [...]
  [ 4136.442494] 3ca0: ffffff94ab7ad584 0000000060400145 ffffffe3a01bf8f8 0000000000000006
  [ 4136.460936] 3cc0: 0000008000000000 ffffff94ab844204 ffffff801d4e3cf0 ffffff94ab7ad584
  [ 4136.479241] [<ffffff94ab7ad584>] bpf_prog_add+0x20/0x68
  [ 4136.491767] [<ffffff94ab7ad638>] bpf_prog_inc+0x20/0x2c
  [ 4136.504536] [<ffffff94ab7b5d08>] bpf_obj_get_user+0x204/0x22c
  [ 4136.518746] [<ffffff94ab7ade68>] SyS_bpf+0x5a8/0x1a88

Android's netd was basically pinning the uid cookie BPF map in BPF
fs (/sys/fs/bpf/traffic_cookie_uid_map) and later on retrieving it
again resulting in above panic. Issue is that the map was wrongly
identified as a prog.

Above kernel was compiled with clang 4.0.3, and it turns out that
clang decided to merge the bpf_prog_iops and bpf_map_iops into a
single memory location, such that the two i_ops could then not be
distinguished anymore.

Reason for this miscompilation is that clang has the more aggressive
-fmerge-all-constants enabled by default. In fact, clang source code
has an 'insightful' comment about it in its source code under
lib/AST/ExprConstant.cpp:

  // Pointers with different bases cannot represent the same object.
  // (Note that clang defaults to -fmerge-all-constants, which can
  // lead to inconsistent results for comparisons involving the address
  // of a constant; this generally doesn't matter in practice.)

gcc on the other hand does not enable -fmerge-all-constants by default
and *explicitly* states in it's option description that using this
flag results in non-conforming behavior, quote from man gcc:

  Languages like C or C++ require each variable, including multiple
  instances of the same variable in recursive calls, to have distinct
  locations, so using this option results in non-conforming behavior.

Given there are users with clang/LLVM out there today that triggered
this, fix this mess by explicitly adding -fno-merge-all-constants to
inode.o as CFLAGS via Kbuild system. Also add a BUILD_BUG_ON() to
bail out when the two are the same address. Potentially we might want
to go even further and do this for the whole kernel as next step,
although given 4.16-rc6, it may be more suited to start out with this
in 4.17.

Reported-by: Prasad Sodagudi <psodagud@codeaurora.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Prasad Sodagudi <psodagud@codeaurora.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/Makefile | 7 +++++++
 kernel/bpf/inode.c  | 1 +
 2 files changed, 8 insertions(+)

diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index a713fd2..8950241 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -1,6 +1,13 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-y := core.o
 
+# Mainly clang workaround that sets this by default. This
+# cannot be used here at all, otherwise it horribly breaks
+# inode ops for maps/progs. gcc does not set this flag by
+# default.
+CFLAGS_REMOVE_inode.o     := -fmerge-all-constants
+CFLAGS_inode.o            := $(call cc-option,-fno-merge-all-constants)
+
 obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
 obj-$(CONFIG_BPF_SYSCALL) += disasm.o
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 81e2f69..ad95360 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -527,6 +527,7 @@ static int __init bpf_init(void)
 	if (ret)
 		sysfs_remove_mount_point(fs_kobj, "bpf");
 
+	BUILD_BUG_ON(&bpf_prog_iops == &bpf_map_iops);
 	return ret;
 }
 fs_initcall(bpf_init);
-- 
2.9.5