Message ID | 20220520165705.2140042-1-keescook@chromium.org (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | KVM: x86/emulator: Bounds check reg nr against reg array size | expand |
On Fri, May 20, 2022, Kees Cook wrote: > GCC 12 sees that it might be possible for "nr" to be outside the _regs > array. Add explicit bounds checking. I think GCC 12 is wrong. There are four uses of reg_rmw() that don't use hardcoded registers: $ git grep reg_rmw | grep -v VCPU_REGS_ emulate.c:static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr) 1 emulate.c: ulong *preg = reg_rmw(ctxt, reg); 2 emulate.c: p = (unsigned char *)reg_rmw(ctxt, modrm_reg & 3) + 1; 3 emulate.c: p = reg_rmw(ctxt, modrm_reg); 4 emulate.c: assign_register(reg_rmw(ctxt, reg), val, ctxt->op_bytes); #1 has three users, but two of those use hardcoded registers. $ git grep register_address_increment | grep -v VCPU_REGS_ emulate.c:register_address_increment(struct x86_emulate_ctxt *ctxt, int reg, int inc) emulate.c: register_address_increment(ctxt, reg, df * op->bytes); and that last one is string_addr_inc(), which is only called with RDI or RSI. #2 can't overflow as the register can only be 0-3 (yay AH/BH/CH/DH operands). #3 is the !highbyte path of decode_register(), and is a bit messy, but modrm_reg is always sanitized. $ git grep -E "decode_register\(" emulate.c:static void *decode_register(struct x86_emulate_ctxt *ctxt, u8 modrm_reg, a emulate.c: op->addr.reg = decode_register(ctxt, reg, ctxt->d & ByteOp); b emulate.c: op->addr.reg = decode_register(ctxt, ctxt->modrm_rm, c emulate.c: ctxt->memop.addr.reg = decode_register(ctxt, ctxt->modrm_rm, true); For (b) and (c), modrm_reg == ctxt->modrm_rm, which is computed in one place and is bounded to 0-15: base_reg = (ctxt->rex_prefix << 3) & 8; /* REX.B */ ctxt->modrm_rm = base_reg | (ctxt->modrm & 0x07); For (a), "reg" is either modrm_reg or a register that is encoded in the opcode, both of which are again bounded to 0-15: unsigned reg = ctxt->modrm_reg; if (!(ctxt->d & ModRM)) reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3); and ctxt->modrm_reg = ((ctxt->rex_prefix << 1) & 8); /* REX.R */ ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3; #4 is em_popa() and is just funky hardcoding of popping RAX-RDI, minus RSP. I did the same exercise for reg_reg() and write_reg(), and the handful of non-hardcoded use are all bounded in similar ways. > In function 'reg_read', > inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: Is there more of the "stack" available? I don't mind the WARN too much, but if there is a bug lurking I would much rather fix the bug. > ../arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array bounds of 'long unsigned int[17]' [-Warray-bounds] > 254 | return ctxt->_regs[nr]; > | ~~~~~~~~~~~^~~~ > In file included from ../arch/x86/kvm/emulate.c:23: > ../arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw': > ../arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs' > 366 | unsigned long _regs[NR_VCPU_REGS]; > | ^~~~~
On Fri, May 20, 2022 at 05:32:04PM +0000, Sean Christopherson wrote: > On Fri, May 20, 2022, Kees Cook wrote: > > GCC 12 sees that it might be possible for "nr" to be outside the _regs > > array. Add explicit bounds checking. > > I think GCC 12 is wrong. I think it's more like GCC is extremely conservative about these things, and assumes the worst when, for whatever reason, it can't track something. > There are four uses of reg_rmw() that don't use hardcoded registers: > > $ git grep reg_rmw | grep -v VCPU_REGS_ > emulate.c:static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr) > 1 emulate.c: ulong *preg = reg_rmw(ctxt, reg); > 2 emulate.c: p = (unsigned char *)reg_rmw(ctxt, modrm_reg & 3) + 1; > 3 emulate.c: p = reg_rmw(ctxt, modrm_reg); > 4 emulate.c: assign_register(reg_rmw(ctxt, reg), val, ctxt->op_bytes); > > #1 has three users, but two of those use hardcoded registers. > > $ git grep register_address_increment | grep -v VCPU_REGS_ > emulate.c:register_address_increment(struct x86_emulate_ctxt *ctxt, int reg, int inc) > emulate.c: register_address_increment(ctxt, reg, df * op->bytes); > > and that last one is string_addr_inc(), which is only called with RDI or RSI. > > #2 can't overflow as the register can only be 0-3 (yay AH/BH/CH/DH operands). > > #3 is the !highbyte path of decode_register(), and is a bit messy, but modrm_reg > is always sanitized. > > $ git grep -E "decode_register\(" > emulate.c:static void *decode_register(struct x86_emulate_ctxt *ctxt, u8 modrm_reg, > a emulate.c: op->addr.reg = decode_register(ctxt, reg, ctxt->d & ByteOp); > b emulate.c: op->addr.reg = decode_register(ctxt, ctxt->modrm_rm, > c emulate.c: ctxt->memop.addr.reg = decode_register(ctxt, > ctxt->modrm_rm, true); > > For (b) and (c), modrm_reg == ctxt->modrm_rm, which is computed in one place and > is bounded to 0-15: > > base_reg = (ctxt->rex_prefix << 3) & 8; /* REX.B */ > ctxt->modrm_rm = base_reg | (ctxt->modrm & 0x07); > > For (a), "reg" is either modrm_reg or a register that is encoded in the opcode, > both of which are again bounded to 0-15: > > unsigned reg = ctxt->modrm_reg; > > if (!(ctxt->d & ModRM)) > reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3); > > and > > ctxt->modrm_reg = ((ctxt->rex_prefix << 1) & 8); /* REX.R */ > ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3; > > #4 is em_popa() and is just funky hardcoding of popping RAX-RDI, minus RSP. > > I did the same exercise for reg_reg() and write_reg(), and the handful of > non-hardcoded use are all bounded in similar ways. Thanks for digging into this. I tried to do the same and started to lose track of things. > > > In function 'reg_read', > > inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: > > Is there more of the "stack" available? I don't mind the WARN too much, but if > there is a bug lurking I would much rather fix the bug. Agreed, but I haven't found a way to get more context here. I think I found a separate place where GCC really does look to have a bug, as it complains about array usage that is explicitly bounded. :P
On Fri, May 20, 2022, Kees Cook wrote: > GCC 12 sees that it might be possible for "nr" to be outside the _regs > array. Add explicit bounds checking. > > In function 'reg_read', > inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: > ../arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array bounds of 'long unsigned int[17]' [-Warray-bounds] > 254 | return ctxt->_regs[nr]; > | ~~~~~~~~~~~^~~~ > In file included from ../arch/x86/kvm/emulate.c:23: > ../arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw': > ../arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs' > 366 | unsigned long _regs[NR_VCPU_REGS]; > | ^~~~~ > > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Sean Christopherson <seanjc@google.com> > Cc: Vitaly Kuznetsov <vkuznets@redhat.com> > Cc: Wanpeng Li <wanpengli@tencent.com> > Cc: Jim Mattson <jmattson@google.com> > Cc: x86@kernel.org > Cc: "H. Peter Anvin" <hpa@zytor.com> > Cc: kvm@vger.kernel.org > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > arch/x86/kvm/emulate.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index 89b11e7dca8a..fbcbc012a3ae 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -247,6 +247,8 @@ enum x86_transfer_type { > > static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) > { > + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) > + return 0; > if (!(ctxt->regs_valid & (1 << nr))) { > ctxt->regs_valid |= 1 << nr; > ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr); > @@ -256,6 +258,8 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) > > static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr) > { > + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) > + return 0; This is wrong, reg_write() confusingly returns a pointer the register to be written, it doesn't actually do the write. So if we want to guard against array overflow, it would be better to cap @nr and continue on, i.e. assume some higher bit was spuriously set. The other oddity here is that VCPU_REGS_RIP should never be read, the RIP relative code reads _eip directly. I.e. _regs[] should really be VCPU_REGS_R15+1. And adding a #define for that would clean up this bit of code in writeback_registers() that hardcodes 16 (rax - r15) GPRs: for_each_set_bit(reg, (ulong *)&ctxt->regs_dirty, 16) ctxt->ops->write_gpr(ctxt, reg, ctxt->_regs[reg]); Lastly, casting regs_dirty to an unsigned long pointer is all kinds of gross, e.g. if it were moved to the end of struct x86_emulate_ctxt then the above could trigger an out-of-bounds read. I'll whip up a small series to clean this code up and add WARNs similar to above.
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index 89b11e7dca8a..fbcbc012a3ae 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -247,6 +247,8 @@ enum x86_transfer_type { static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) { + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) + return 0; if (!(ctxt->regs_valid & (1 << nr))) { ctxt->regs_valid |= 1 << nr; ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr); @@ -256,6 +258,8 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr) { + if (WARN_ON(nr >= ARRAY_SIZE(ctxt->_regs))) + return 0; ctxt->regs_valid |= 1 << nr; ctxt->regs_dirty |= 1 << nr; return &ctxt->_regs[nr];
GCC 12 sees that it might be possible for "nr" to be outside the _regs array. Add explicit bounds checking. In function 'reg_read', inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2: ../arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array bounds of 'long unsigned int[17]' [-Warray-bounds] 254 | return ctxt->_regs[nr]; | ~~~~~~~~~~~^~~~ In file included from ../arch/x86/kvm/emulate.c:23: ../arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw': ../arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs' 366 | unsigned long _regs[NR_VCPU_REGS]; | ^~~~~ Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: Vitaly Kuznetsov <vkuznets@redhat.com> Cc: Wanpeng Li <wanpengli@tencent.com> Cc: Jim Mattson <jmattson@google.com> Cc: x86@kernel.org Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: kvm@vger.kernel.org Signed-off-by: Kees Cook <keescook@chromium.org> --- arch/x86/kvm/emulate.c | 4 ++++ 1 file changed, 4 insertions(+)