From bd5a410a5de3a6893eaacc749e706b85506dc908 Mon Sep 17 00:00:00 2001 From: Ricardo Neri Date: Fri, 27 Oct 2017 13:25:42 -0700 Subject: [PATCH] x86/insn-eval: Add utility functions to get segment descriptor base address and limit With segmentation, the base address of the segment is needed to compute a linear address. This base address is obtained from the applicable segment descriptor. Such segment descriptor is referenced from a segment selector. These new functions obtain the segment base and limit of the segment selector indicated by segment register index given as argument. This index is any of the INAT_SEG_REG_* family of #define's. The logic to obtain the segment selector is wrapped in the function get_segment_selector() with the inputs described above. Once the selector is known, the base address is determined. In protected mode, the selector is used to obtain the segment descriptor and then its base address. In long mode, the segment base address is zero except when FS or GS are used. In virtual-8086 mode, the base address is computed as the value of the segment selector shifted 4 positions to the left. In protected mode, segment limits are enforced. Thus, a function to determine the limit of the segment is added. Segment limits are not enforced in long or virtual-8086. For the latter, addresses are limited to 20 bits; address size will be handled when computing the linear address. Improvements-by: Borislav Petkov Signed-off-by: Ricardo Neri Signed-off-by: Thomas Gleixner Reviewed-by: Borislav Petkov Cc: "Michael S. Tsirkin" Cc: Peter Zijlstra Cc: Dave Hansen Cc: ricardo.neri@intel.com Cc: Adrian Hunter Cc: Paul Gortmaker Cc: Huang Rui Cc: Qiaowei Ren Cc: Shuah Khan Cc: Kees Cook Cc: Jonathan Corbet Cc: Jiri Slaby Cc: Dmitry Vyukov Cc: "Ravi V. Shankar" Cc: Chris Metcalf Cc: Brian Gerst Cc: Arnaldo Carvalho de Melo Cc: Andy Lutomirski Cc: Colin Ian King Cc: Chen Yucong Cc: Adam Buchbinder Cc: Vlastimil Babka Cc: Lorenzo Stoakes Cc: Masami Hiramatsu Cc: Paolo Bonzini Cc: Andrew Morton Cc: Thomas Garnier Link: https://lkml.kernel.org/r/1509135945-13762-16-git-send-email-ricardo.neri-calderon@linux.intel.com --- arch/x86/include/asm/insn-eval.h | 1 + arch/x86/lib/insn-eval.c | 114 +++++++++++++++++++++++++++++++ 2 files changed, 115 insertions(+) diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h index 7e8c9633a377..25d6e44dcbfc 100644 --- a/arch/x86/include/asm/insn-eval.h +++ b/arch/x86/include/asm/insn-eval.h @@ -13,5 +13,6 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs); int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs); +unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx); #endif /* _ASM_X86_INSN_EVAL_H */ diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c index d85e84091ddc..89d5c8902f42 100644 --- a/arch/x86/lib/insn-eval.c +++ b/arch/x86/lib/insn-eval.c @@ -525,6 +525,120 @@ static struct desc_struct *get_desc(unsigned short sel) return (struct desc_struct *)(gdt_desc.address + desc_base); } +/** + * insn_get_seg_base() - Obtain base address of segment descriptor. + * @regs: Register values as seen when entering kernel mode + * @seg_reg_idx: Index of the segment register pointing to seg descriptor + * + * Obtain the base address of the segment as indicated by the segment descriptor + * pointed by the segment selector. The segment selector is obtained from the + * input segment register index @seg_reg_idx. + * + * Returns: + * + * In protected mode, base address of the segment. Zero in long mode, + * except when FS or GS are used. In virtual-8086 mode, the segment + * selector shifted 4 bits to the right. + * + * -1L in case of error. + */ +unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx) +{ + struct desc_struct *desc; + short sel; + + sel = get_segment_selector(regs, seg_reg_idx); + if (sel < 0) + return -1L; + + if (v8086_mode(regs)) + /* + * Base is simply the segment selector shifted 4 + * bits to the right. + */ + return (unsigned long)(sel << 4); + + if (user_64bit_mode(regs)) { + /* + * Only FS or GS will have a base address, the rest of + * the segments' bases are forced to 0. + */ + unsigned long base; + + if (seg_reg_idx == INAT_SEG_REG_FS) + rdmsrl(MSR_FS_BASE, base); + else if (seg_reg_idx == INAT_SEG_REG_GS) + /* + * swapgs was called at the kernel entry point. Thus, + * MSR_KERNEL_GS_BASE will have the user-space GS base. + */ + rdmsrl(MSR_KERNEL_GS_BASE, base); + else + base = 0; + return base; + } + + /* In protected mode the segment selector cannot be null. */ + if (!sel) + return -1L; + + desc = get_desc(sel); + if (!desc) + return -1L; + + return get_desc_base(desc); +} + +/** + * get_seg_limit() - Obtain the limit of a segment descriptor + * @regs: Register values as seen when entering kernel mode + * @seg_reg_idx: Index of the segment register pointing to seg descriptor + * + * Obtain the limit of the segment as indicated by the segment descriptor + * pointed by the segment selector. The segment selector is obtained from the + * input segment register index @seg_reg_idx. + * + * Returns: + * + * In protected mode, the limit of the segment descriptor in bytes. + * In long mode and virtual-8086 mode, segment limits are not enforced. Thus, + * limit is returned as -1L to imply a limit-less segment. + * + * Zero is returned on error. + */ +static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx) +{ + struct desc_struct *desc; + unsigned long limit; + short sel; + + sel = get_segment_selector(regs, seg_reg_idx); + if (sel < 0) + return 0; + + if (user_64bit_mode(regs) || v8086_mode(regs)) + return -1L; + + if (!sel) + return 0; + + desc = get_desc(sel); + if (!desc) + return 0; + + /* + * If the granularity bit is set, the limit is given in multiples + * of 4096. This also means that the 12 least significant bits are + * not tested when checking the segment limits. In practice, + * this means that the segment ends in (limit << 12) + 0xfff. + */ + limit = get_desc_limit(desc); + if (desc->g) + limit = (limit << 12) + 0xfff; + + return limit; +} + /** * insn_get_modrm_rm_off() - Obtain register in r/m part of the ModRM byte * @insn: Instruction containing the ModRM byte -- 2.39.5