From: Andrew Cooper Subject: x86/segment: Bounds check accesses to emulation ctxt->seg_reg[] HVM HAP codepaths have space for all segment registers in the seg_reg[] cache (with x86_seg_none still risking an array overrun), while the shadow codepaths only have space for the user segments. Range check the input segment of *_get_seg_reg() against the size of the array used to cache the results, to avoid overruns in the case that the callers don't filter their input suitably. Subsume the is_x86_user_segment(seg) checks from the shadow code, which were an incomplete attempt at range checking, and are now superceeded. Make hvm_get_seg_reg() static, as it is not used outside of shadow/common.c No functional change, but far easier to reason that no overflow is possible. Reported-by: Andrew Cooper Signed-off-by: Andrew Cooper Acked-by: Tim Deegan Acked-by: Jan Beulich --- a/xen/arch/x86/hvm/emulate.c +++ b/xen/arch/x86/hvm/emulate.c @@ -534,6 +534,8 @@ static int hvmemul_virtual_to_linear( *reps = min_t(unsigned long, *reps, max_reps); reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt); + if ( IS_ERR(reg) ) + return -PTR_ERR(reg); if ( (hvmemul_ctxt->ctxt.regs->eflags & X86_EFLAGS_DF) && (*reps > 1) ) { @@ -1369,6 +1371,10 @@ static int hvmemul_read_segment( struct hvm_emulate_ctxt *hvmemul_ctxt = container_of(ctxt, struct hvm_emulate_ctxt, ctxt); struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt); + + if ( IS_ERR(sreg) ) + return -PTR_ERR(sreg); + memcpy(reg, sreg, sizeof(struct segment_register)); return X86EMUL_OKAY; } @@ -1382,6 +1388,9 @@ static int hvmemul_write_segment( container_of(ctxt, struct hvm_emulate_ctxt, ctxt); struct segment_register *sreg = hvmemul_get_seg_reg(seg, hvmemul_ctxt); + if ( IS_ERR(sreg) ) + return -PTR_ERR(sreg); + memcpy(sreg, reg, sizeof(struct segment_register)); __set_bit(seg, &hvmemul_ctxt->seg_reg_dirty); @@ -1934,10 +1943,17 @@ void hvm_emulate_writeback( } } +/* + * Callers which pass a known in-range x86_segment can rely on the return + * pointer being valid. Other callers must explicitly check for errors. + */ struct segment_register *hvmemul_get_seg_reg( enum x86_segment seg, struct hvm_emulate_ctxt *hvmemul_ctxt) { + if ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) ) + return ERR_PTR(-X86EMUL_UNHANDLEABLE); + if ( !__test_and_set_bit(seg, &hvmemul_ctxt->seg_reg_accessed) ) hvm_get_segment_register(current, seg, &hvmemul_ctxt->seg_reg[seg]); return &hvmemul_ctxt->seg_reg[seg]; --- a/xen/arch/x86/mm/shadow/common.c +++ b/xen/arch/x86/mm/shadow/common.c @@ -123,10 +123,19 @@ __initcall(shadow_audit_key_init); /* x86 emulator support for the shadow code */ -struct segment_register *hvm_get_seg_reg( +/* + * Callers which pass a known in-range x86_segment can rely on the return + * pointer being valid. Other callers must explicitly check for errors. + */ +static struct segment_register *hvm_get_seg_reg( enum x86_segment seg, struct sh_emulate_ctxt *sh_ctxt) { - struct segment_register *seg_reg = &sh_ctxt->seg_reg[seg]; + struct segment_register *seg_reg; + + if ( seg < 0 || seg >= ARRAY_SIZE(sh_ctxt->seg_reg) ) + return ERR_PTR(-X86EMUL_UNHANDLEABLE); + + seg_reg = &sh_ctxt->seg_reg[seg]; if ( !__test_and_set_bit(seg, &sh_ctxt->valid_seg_regs) ) hvm_get_segment_register(current, seg, seg_reg); return seg_reg; @@ -143,14 +152,9 @@ static int hvm_translate_linear_addr( struct segment_register *reg; int okay; - /* - * Can arrive here with non-user segments. However, no such cirucmstance - * is part of a legitimate pagetable update, so fail the emulation. - */ - if ( !is_x86_user_segment(seg) ) - return X86EMUL_UNHANDLEABLE; - reg = hvm_get_seg_reg(seg, sh_ctxt); + if ( IS_ERR(reg) ) + return -PTR_ERR(reg); okay = hvm_virtual_to_linear_addr( seg, reg, offset, bytes, access_type, sh_ctxt->ctxt.addr_size, paddr); @@ -253,9 +257,6 @@ hvm_emulate_write(enum x86_segment seg, unsigned long addr; int rc; - if ( !is_x86_user_segment(seg) ) - return X86EMUL_UNHANDLEABLE; - /* How many emulations could we save if we unshadowed on stack writes? */ if ( seg == x86_seg_ss ) perfc_incr(shadow_fault_emulate_stack); @@ -283,7 +284,7 @@ hvm_emulate_cmpxchg(enum x86_segment seg unsigned long addr, old, new; int rc; - if ( !is_x86_user_segment(seg) || bytes > sizeof(long) ) + if ( bytes > sizeof(long) ) return X86EMUL_UNHANDLEABLE; rc = hvm_translate_linear_addr( --- a/xen/arch/x86/mm/shadow/private.h +++ b/xen/arch/x86/mm/shadow/private.h @@ -740,8 +740,6 @@ const struct x86_emulate_ops *shadow_ini struct sh_emulate_ctxt *sh_ctxt, struct cpu_user_regs *regs); void shadow_continue_emulation( struct sh_emulate_ctxt *sh_ctxt, struct cpu_user_regs *regs); -struct segment_register *hvm_get_seg_reg( - enum x86_segment seg, struct sh_emulate_ctxt *sh_ctxt); #if (SHADOW_OPTIMIZATIONS & SHOPT_VIRTUAL_TLB) /**************************************************************************/ --- a/xen/include/asm-x86/hvm/emulate.h +++ b/xen/include/asm-x86/hvm/emulate.h @@ -13,6 +13,7 @@ #define __ASM_X86_HVM_EMULATE_H__ #include +#include #include #include