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bpf ppc64: Access only if addr is kernel address

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On PPC64 with KUAP enabled, any kernel code which wants to
access userspace needs to be surrounded by disable-enable KUAP.
But that is not happening for BPF_PROBE_MEM load instruction.
So, when BPF program tries to access invalid userspace address,
page-fault handler considers it as bad KUAP fault:

  Kernel attempted to read user page (d0000000) - exploit attempt? (uid: 0)

Considering the fact that PTR_TO_BTF_ID (which uses BPF_PROBE_MEM
mode) could either be a valid kernel pointer or NULL but should
never be a pointer to userspace address, execute BPF_PROBE_MEM load
only if addr is kernel address, otherwise set dst_reg=0 and move on.

This will catch NULL, valid or invalid userspace pointers. Only bad
kernel pointer will be handled by BPF exception table.

[Alexei suggested for x86]

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211012123056.485795-7-hbathini@linux.ibm.com
This commit is contained in:
Ravi Bangoria 2021-10-12 18:00:54 +05:30 committed by Michael Ellerman
parent 983bdc0245
commit 9c70c7147f
1 changed files with 26 additions and 0 deletions
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26
arch/powerpc/net/bpf_jit_comp64.c
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@ -789,6 +789,32 @@ emit_clear:
/* dst = *(u64 *)(ul) (src + off) */
case BPF_LDX | BPF_MEM | BPF_DW:
case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
/*
* As PTR_TO_BTF_ID that uses BPF_PROBE_MEM mode could either be a valid
* kernel pointer or NULL but not a userspace address, execute BPF_PROBE_MEM
* load only if addr is kernel address (see is_kernel_addr()), otherwise
* set dst_reg=0 and move on.
*/
if (BPF_MODE(code) == BPF_PROBE_MEM) {
EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
if (IS_ENABLED(CONFIG_PPC_BOOK3E_64))
PPC_LI64(b2p[TMP_REG_2], 0x8000000000000000ul);
else /* BOOK3S_64 */
PPC_LI64(b2p[TMP_REG_2], PAGE_OFFSET);
EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
EMIT(PPC_RAW_LI(dst_reg, 0));
/*
* Check if 'off' is word aligned because PPC_BPF_LL()
* (BPF_DW case) generates two instructions if 'off' is not
* word-aligned and one instruction otherwise.
*/
if (BPF_SIZE(code) == BPF_DW && (off & 3))
PPC_JMP((ctx->idx + 3) * 4);
else
PPC_JMP((ctx->idx + 2) * 4);
}
switch (size) {
case BPF_B:
EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));