On 2024/6/20 14:46, Leon Hwang wrote: > > > On 20/6/24 14:07, Tengda Wu wrote: >> When loading a EXT program without specifying `attr->attach_prog_fd`, >> the `prog->aux->dst_prog` will be null. At this time, calling >> resolve_prog_type() anywhere will result in a null pointer dereference. > > Interesting, same NULL pointer dereference causes another issue[0]. > > As for my case, when resolve_prog_type(), it has to use > prog->aux->saved_dst_prog_type instead of prog->aux->dst_prog->type for > EXT program, in order to avoid NULL pointer dereference. > > [0] https://lore.kernel.org/bpf/20240602122421.50892-2-hffilwlqm@xxxxxxxxx/ > > Thanks, > Leon >This looks good, but unfortunately, there is still a problem with using `prog->aux->saved_dst_prog_type` to resolve prog type, because its value still comes from `prog->aux->dst_prog`in check_attach_btf_id(). Additionally, resolve_prog_type() not always be used after check_attach_btf_id(). The following example stack trace proves the existence of this situation. It shows that NULL pointer dereference occurs in add_subprog_and_kfunc(), which check_attach_btf_id() has not yet reached. So it may be more effective to check and avoid dst_prog empty when EXT program loads. >> >> Example stack trace: >> >> [ 8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004 >> [ 8.108262] Mem abort info: >> [ 8.108384] ESR = 0x0000000096000004 >> [ 8.108547] EC = 0x25: DABT (current EL), IL = 32 bits >> [ 8.108722] SET = 0, FnV = 0 >> [ 8.108827] EA = 0, S1PTW = 0 >> [ 8.108939] FSC = 0x04: level 0 translation fault >> [ 8.109102] Data abort info: >> [ 8.109203] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000 >> [ 8.109399] CM = 0, WnR = 0, TnD = 0, TagAccess = 0 >> [ 8.109614] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 >> [ 8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000 >> [ 8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000 >> [ 8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP >> [ 8.112783] Modules linked in: >> [ 8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1 >> [ 8.113230] Hardware name: linux,dummy-virt (DT) >> [ 8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) >> [ 8.113429] pc : may_access_direct_pkt_data+0x24/0xa0 >> [ 8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8 >> [ 8.113798] sp : ffff80008283b9f0 >> [ 8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001 >> [ 8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000 >> [ 8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000 >> [ 8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff >> [ 8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720 >> [ 8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720 >> [ 8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4 >> [ 8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f >> [ 8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c >> [ 8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000 >> [ 8.114126] Call trace: >> [ 8.114159] may_access_direct_pkt_data+0x24/0xa0 >> [ 8.114202] bpf_check+0x3bc/0x28c0 >> [ 8.114214] bpf_prog_load+0x658/0xa58 >> [ 8.114227] __sys_bpf+0xc50/0x2250 >> [ 8.114240] __arm64_sys_bpf+0x28/0x40 >> [ 8.114254] invoke_syscall.constprop.0+0x54/0xf0 >> [ 8.114273] do_el0_svc+0x4c/0xd8 >> [ 8.114289] el0_svc+0x3c/0x140 >> [ 8.114305] el0t_64_sync_handler+0x134/0x150 >> [ 8.114331] el0t_64_sync+0x168/0x170 >> [ 8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403) >> [ 8.118672] ---[ end trace 0000000000000000 ]--- >> >> Fix this by adding dst_prog non-empty check in BPF_PROG_TYPE_EXT case >> when calling bpf_prog_load(). >> >> Fixes: 4a9c7bbe2ed4 ("bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT") >> Signed-off-by: Tengda Wu <wutengda@xxxxxxxxxxxxxxx> >> Cc: stable@xxxxxxxxxxxxxxx # v5.18+ >> --- >> kernel/bpf/syscall.c | 5 ++++- >> 1 file changed, 4 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c >> index f45ed6adc092..4490f8ccf006 100644 >> --- a/kernel/bpf/syscall.c >> +++ b/kernel/bpf/syscall.c >> @@ -2632,9 +2632,12 @@ bpf_prog_load_check_attach(enum bpf_prog_type prog_type, >> return 0; >> return -EINVAL; >> case BPF_PROG_TYPE_SYSCALL: >> - case BPF_PROG_TYPE_EXT: >> if (expected_attach_type) >> return -EINVAL; >> + return 0; >> + case BPF_PROG_TYPE_EXT: >> + if (expected_attach_type || !dst_prog) >> + return -EINVAL; >> fallthrough; >> default: >> return 0;
