From 773cd38f40b8834be991dbfed36683acc1dd41ee Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Tue, 13 May 2014 15:05:55 -0700 Subject: [PATCH] net: filter: x86: fix JIT address randomization bpf_alloc_binary() adds 128 bytes of room to JITed program image and rounds it up to the nearest page size. If image size is close to page size (like 4000), it is rounded to two pages: round_up(4000 + 4 + 128) == 8192 then 'hole' is computed as 8192 - (4000 + 4) = 4188 If prandom_u32() % hole selects a number >= PAGE_SIZE - sizeof(*header) then kernel will crash during bpf_jit_free(): kernel BUG at arch/x86/mm/pageattr.c:887! Call Trace: [] change_page_attr_set_clr+0x135/0x460 [] ? _raw_spin_unlock_irq+0x30/0x50 [] set_memory_rw+0x2f/0x40 [] bpf_jit_free_deferred+0x2d/0x60 [] process_one_work+0x1d8/0x6a0 [] ? process_one_work+0x178/0x6a0 [] worker_thread+0x11c/0x370 since bpf_jit_free() does: unsigned long addr = (unsigned long)fp->bpf_func & PAGE_MASK; struct bpf_binary_header *header = (void *)addr; to compute start address of 'bpf_binary_header' and header->pages will pass junk to: set_memory_rw(addr, header->pages); Fix it by making sure that &header->image[prandom_u32() % hole] and &header are in the same page Fixes: 314beb9bcabfd ("x86: bpf_jit_comp: secure bpf jit against spraying attacks") Signed-off-by: Alexei Starovoitov Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- arch/x86/net/bpf_jit_comp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index dc017735bb91..6d5663a599a7 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -171,7 +171,7 @@ static struct bpf_binary_header *bpf_alloc_binary(unsigned int proglen, memset(header, 0xcc, sz); /* fill whole space with int3 instructions */ header->pages = sz / PAGE_SIZE; - hole = sz - (proglen + sizeof(*header)); + hole = min(sz - (proglen + sizeof(*header)), PAGE_SIZE - sizeof(*header)); /* insert a random number of int3 instructions before BPF code */ *image_ptr = &header->image[prandom_u32() % hole]; -- 2.20.1