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Commit 0bae2d4d authored by Jiong Wang's avatar Jiong Wang Committed by Alexei Starovoitov
Browse files

bpf: correct slot_type marking logic to allow more stack slot sharing 



Verifier is supposed to support sharing stack slot allocated to ptr with
SCALAR_VALUE for privileged program. However this doesn't happen for some
cases.

The reason is verifier is not clearing slot_type STACK_SPILL for all bytes,
it only clears part of them, while verifier is using:

  slot_type[0] == STACK_SPILL

as a convention to check one slot is ptr type.

So, the consequence of partial clearing slot_type is verifier could treat a
partially overridden ptr slot, which should now be a SCALAR_VALUE slot,
still as ptr slot, and rejects some valid programs.

Before this patch, test_xdp_noinline.o under bpf selftests, bpf_lxc.o and
bpf_netdev.o under Cilium bpf repo, when built with -mattr=+alu32 are
rejected due to this issue. After this patch, they all accepted.

There is no processed insn number change before and after this patch on
Cilium bpf programs.

Reviewed-by: default avatarJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: default avatarJiong Wang <jiong.wang@netronome.com>
Reviewed-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent a38d1107

kernel/bpf/verifier.c

+5 −0
Original line number Diff line number Diff line
@@ -1286,6 +1286,10 @@ static int check_stack_write(struct bpf_verifier_env *env, 


		/* regular write of data into stack destroys any spilled ptr */

		state->stack[spi].spilled_ptr.type = NOT_INIT;

		/* Mark slots as STACK_MISC if they belonged to spilled ptr. */

		if (state->stack[spi].slot_type[0] == STACK_SPILL)

			for (i = 0; i < BPF_REG_SIZE; i++)

				state->stack[spi].slot_type[i] = STACK_MISC;



		/* only mark the slot as written if all 8 bytes were written

		 * otherwise read propagation may incorrectly stop too soon
@@ -1303,6 +1307,7 @@ static int check_stack_write(struct bpf_verifier_env *env, 
		    register_is_null(&cur->regs[value_regno]))

			type = STACK_ZERO;



		/* Mark slots affected by this stack write. */

		for (i = 0; i < size; i++)

			state->stack[spi].slot_type[(slot - i) % BPF_REG_SIZE] =

				type;

tools/testing/selftests/bpf/test_verifier.c

+32 −2
Original line number Diff line number Diff line
@@ -1001,14 +1001,44 @@ static struct bpf_test tests[] = { 
			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),

			/* mess up with R1 pointer on stack */

			BPF_ST_MEM(BPF_B, BPF_REG_10, -7, 0x23),

			/* fill back into R0 should fail */

			/* fill back into R0 is fine for priv.

			 * R0 now becomes SCALAR_VALUE.

			 */

			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),

			/* Load from R0 should fail. */

			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 8),

			BPF_EXIT_INSN(),

		},

		.errstr_unpriv = "attempt to corrupt spilled",

		.errstr = "corrupted spill",

		.errstr = "R0 invalid mem access 'inv",

		.result = REJECT,

	},

	{

		"check corrupted spill/fill, LSB",

		.insns = {

			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),

			BPF_ST_MEM(BPF_H, BPF_REG_10, -8, 0xcafe),

			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),

			BPF_EXIT_INSN(),

		},

		.errstr_unpriv = "attempt to corrupt spilled",

		.result_unpriv = REJECT,

		.result = ACCEPT,

		.retval = POINTER_VALUE,

	},

	{

		"check corrupted spill/fill, MSB",

		.insns = {

			BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_1, -8),

			BPF_ST_MEM(BPF_W, BPF_REG_10, -4, 0x12345678),

			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),

			BPF_EXIT_INSN(),

		},

		.errstr_unpriv = "attempt to corrupt spilled",

		.result_unpriv = REJECT,

		.result = ACCEPT,

		.retval = POINTER_VALUE,

	},

	{

		"invalid src register in STX",

		.insns = {