| Message ID | 20240627170900.1672542-4-andrii@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | ioctl()-based API to query VMAs from /proc/<pid>/maps | expand |
Andrii Nakryiko <andrii@kernel.org> writes: > The need to get ELF build ID reliably is an important aspect when > dealing with profiling and stack trace symbolization, and > /proc/<pid>/maps textual representation doesn't help with this. > > To get backing file's ELF build ID, application has to first resolve > VMA, then use it's start/end address range to follow a special > /proc/<pid>/map_files/<start>-<end> symlink to open the ELF file (this > is necessary because backing file might have been removed from the disk > or was already replaced with another binary in the same file path. > > Such approach, beyond just adding complexity of having to do a bunch of > extra work, has extra security implications. Because application opens > underlying ELF file and needs read access to its entire contents (as far > as kernel is concerned), kernel puts additional capable() checks on > following /proc/<pid>/map_files/<start>-<end> symlink. And that makes > sense in general. I was curious about this statement. It has still certainly potential for side channels e.g. for files that are execute only, or with some other special protection. But actually just looking at the parsing code it seems to fail basic TOCTTOU rules, and since you don't check if the VMA mapping is executable (I think), so there's no EBUSY checking for writes, it likely is exploitable. /* only supports phdr that fits in one page */ if (ehdr->e_phnum > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr)) <---------- check in memory return -EINVAL; phdr = (Elf64_Phdr *)(page_addr + sizeof(Elf64_Ehdr)); <---- but page is shared in the page cache. So if anybody manages to map it for write for (i = 0; i < ehdr->e_phnum; ++i) { <----- this loop can go off into the next page. if (phdr[i].p_type == PT_NOTE && !parse_build_id(page_addr, build_id, size, page_addr + phdr[i].p_offset, phdr[i].p_filesz)) return 0; Here's an untested patch diff --git a/lib/buildid.c b/lib/buildid.c index 7954dd92e36c..6c022fcd03ec 100644 --- a/lib/buildid.c +++ b/lib/buildid.c @@ -72,19 +72,20 @@ static int get_build_id_32(const void *page_addr, unsigned char *build_id, Elf32_Ehdr *ehdr = (Elf32_Ehdr *)page_addr; Elf32_Phdr *phdr; int i; + unsigned phnum = READ_ONCE(ehdr->e_phnum); /* only supports phdr that fits in one page */ - if (ehdr->e_phnum > + if (phnum > (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr)) return -EINVAL; phdr = (Elf32_Phdr *)(page_addr + sizeof(Elf32_Ehdr)); - for (i = 0; i < ehdr->e_phnum; ++i) { + for (i = 0; i < phnum; ++i) { if (phdr[i].p_type == PT_NOTE && !parse_build_id(page_addr, build_id, size, page_addr + phdr[i].p_offset, - phdr[i].p_filesz)) + READ_ONCE(phdr[i].p_filesz))) return 0; } return -EINVAL; @@ -97,15 +98,16 @@ static int get_build_id_64(const void *page_addr, unsigned char *build_id, Elf64_Ehdr *ehdr = (Elf64_Ehdr *)page_addr; Elf64_Phdr *phdr; int i; + unsigned phnum = READ_ONCE(ehdr->e_phnum); /* only supports phdr that fits in one page */ - if (ehdr->e_phnum > + if (phnum > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr)) return -EINVAL; phdr = (Elf64_Phdr *)(page_addr + sizeof(Elf64_Ehdr)); - for (i = 0; i < ehdr->e_phnum; ++i) { + for (i = 0; i < phnum; ++i) { if (phdr[i].p_type == PT_NOTE && !parse_build_id(page_addr, build_id, size, page_addr + phdr[i].p_offset,
On Thu, Jun 27, 2024 at 4:00 PM Andi Kleen <ak@linux.intel.com> wrote: > > Andrii Nakryiko <andrii@kernel.org> writes: > > > The need to get ELF build ID reliably is an important aspect when > > dealing with profiling and stack trace symbolization, and > > /proc/<pid>/maps textual representation doesn't help with this. > > > > To get backing file's ELF build ID, application has to first resolve > > VMA, then use it's start/end address range to follow a special > > /proc/<pid>/map_files/<start>-<end> symlink to open the ELF file (this > > is necessary because backing file might have been removed from the disk > > or was already replaced with another binary in the same file path. > > > > Such approach, beyond just adding complexity of having to do a bunch of > > extra work, has extra security implications. Because application opens > > underlying ELF file and needs read access to its entire contents (as far > > as kernel is concerned), kernel puts additional capable() checks on > > following /proc/<pid>/map_files/<start>-<end> symlink. And that makes > > sense in general. > > I was curious about this statement. It has still certainly potential > for side channels e.g. for files that are execute only, or with > some other special protection. > > But actually just looking at the parsing code it seems to fail basic > TOCTTOU rules, and since you don't check if the VMA mapping is executable > (I think), so there's no EBUSY checking for writes, it likely is exploitable. > > > /* only supports phdr that fits in one page */ > if (ehdr->e_phnum > > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr)) > <---------- check in memory > return -EINVAL; > > phdr = (Elf64_Phdr *)(page_addr + sizeof(Elf64_Ehdr)); > > <---- but page is shared in the page cache. So if anybody manages to map > it for write > > > for (i = 0; i < ehdr->e_phnum; ++i) { <----- this loop can go > off into the next page. > if (phdr[i].p_type == PT_NOTE && > !parse_build_id(page_addr, build_id, size, > page_addr + phdr[i].p_offset, > phdr[i].p_filesz)) > return 0; > > Here's an untested patch > > Yep, makes sense. I'm currently reworking this whole lib/buildid.c implementation to remove all the restrictions on data being in the first page only, and making it work in a faultable context more reliably. I can audit the code for TOCTOU issues and incorporate your feedback. I'll probably post the patch set next week, will cc you as well. > diff --git a/lib/buildid.c b/lib/buildid.c > index 7954dd92e36c..6c022fcd03ec 100644 > --- a/lib/buildid.c > +++ b/lib/buildid.c > @@ -72,19 +72,20 @@ static int get_build_id_32(const void *page_addr, unsigned char *build_id, > Elf32_Ehdr *ehdr = (Elf32_Ehdr *)page_addr; > Elf32_Phdr *phdr; > int i; > + unsigned phnum = READ_ONCE(ehdr->e_phnum); > > /* only supports phdr that fits in one page */ > - if (ehdr->e_phnum > > + if (phnum > > (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr)) > return -EINVAL; > > phdr = (Elf32_Phdr *)(page_addr + sizeof(Elf32_Ehdr)); > > - for (i = 0; i < ehdr->e_phnum; ++i) { > + for (i = 0; i < phnum; ++i) { > if (phdr[i].p_type == PT_NOTE && > !parse_build_id(page_addr, build_id, size, > page_addr + phdr[i].p_offset, > - phdr[i].p_filesz)) > + READ_ONCE(phdr[i].p_filesz))) > return 0; > } > return -EINVAL; > @@ -97,15 +98,16 @@ static int get_build_id_64(const void *page_addr, unsigned char *build_id, > Elf64_Ehdr *ehdr = (Elf64_Ehdr *)page_addr; > Elf64_Phdr *phdr; > int i; > + unsigned phnum = READ_ONCE(ehdr->e_phnum); > > /* only supports phdr that fits in one page */ > - if (ehdr->e_phnum > > + if (phnum > > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr)) > return -EINVAL; > > phdr = (Elf64_Phdr *)(page_addr + sizeof(Elf64_Ehdr)); > > - for (i = 0; i < ehdr->e_phnum; ++i) { > + for (i = 0; i < phnum; ++i) { > if (phdr[i].p_type == PT_NOTE && > !parse_build_id(page_addr, build_id, size, > page_addr + phdr[i].p_offset,
> Yep, makes sense. I'm currently reworking this whole lib/buildid.c > implementation to remove all the restrictions on data being in the > first page only, and making it work in a faultable context more > reliably. I can audit the code for TOCTOU issues and incorporate your > feedback. I'll probably post the patch set next week, will cc you as > well. Please also add checks that the mapping is executable, to close the obscure "can check the first 4 bytes of every mapped file is ELF\0" hole. But it will still need the hardening because mappings from ld.so are not EBUSY for writes. -Andi
On Fri, Jun 28, 2024 at 3:33 PM Andi Kleen <ak@linux.intel.com> wrote: > > > Yep, makes sense. I'm currently reworking this whole lib/buildid.c > > implementation to remove all the restrictions on data being in the > > first page only, and making it work in a faultable context more > > reliably. I can audit the code for TOCTOU issues and incorporate your > > feedback. I'll probably post the patch set next week, will cc you as > > well. > > Please also add checks that the mapping is executable, to > close the obscure "can check the first 4 bytes of every mapped > file is ELF\0" hole. > > But it will still need the hardening because mappings from > ld.so are not EBUSY for writes. I'm a bit confused. Two things: 1) non-executable file-backed VMA still has build ID associated with it. Note, build ID is extracted from the backing file's content, not from VMA itself. The part of ELF file that contains build ID isn't necessarily mmap()'ed at all 2) What sort of exploitation are we talking about here? it's not enough for backing file to have correct 4 starting bytes (0x7f"ELF"), we still have to find correct PT_NOTE segment, and .note.gnu.build-id section within it, that has correct type (3) and key name "GNU". I'm trying to understand what we are protecting against here. Especially that opening /proc/<pid>/maps already requires PTRACE_MODE_READ permissions anyways (or pid should be self). > > -Andi
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 674405b99d0d..32bef3eeab7f 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -22,6 +22,7 @@ #include <linux/pkeys.h> #include <linux/minmax.h> #include <linux/overflow.h> +#include <linux/buildid.h> #include <asm/elf.h> #include <asm/tlb.h> @@ -445,6 +446,7 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm, addr = vma->vm_end; if (flags & PROCMAP_QUERY_COVERING_OR_NEXT_VMA) goto next_vma; + no_vma: return ERR_PTR(-ENOENT); } @@ -455,7 +457,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) struct vm_area_struct *vma; struct mm_struct *mm; const char *name = NULL; - char *name_buf = NULL; + char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL; __u64 usize; int err; @@ -477,6 +479,8 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) /* either both buffer address and size are set, or both should be zero */ if (!!karg.vma_name_size != !!karg.vma_name_addr) return -EINVAL; + if (!!karg.build_id_size != !!karg.build_id_addr) + return -EINVAL; mm = priv->mm; if (!mm || !mmget_not_zero(mm)) @@ -539,6 +543,21 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) } } + if (karg.build_id_size) { + __u32 build_id_sz; + + err = build_id_parse(vma, build_id_buf, &build_id_sz); + if (err) { + karg.build_id_size = 0; + } else { + if (karg.build_id_size < build_id_sz) { + err = -ENAMETOOLONG; + goto out; + } + karg.build_id_size = build_id_sz; + } + } + if (karg.vma_name_size) { size_t name_buf_sz = min_t(size_t, PATH_MAX, karg.vma_name_size); const struct path *path; @@ -583,6 +602,10 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) } kfree(name_buf); + if (karg.build_id_size && copy_to_user((void __user *)karg.build_id_addr, + build_id_buf, karg.build_id_size)) + return -EFAULT; + if (copy_to_user(uarg, &karg, min_t(size_t, sizeof(karg), usize))) return -EFAULT; diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index 5d440f9b5d92..2a4a5f50c98e 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -511,6 +511,26 @@ struct procmap_query { * If set to zero, vma_name_addr should be set to zero as well */ __u32 vma_name_size; /* in/out */ + /* + * If set to non-zero value, signals the request to extract and return + * VMA's backing file's build ID, if the backing file is an ELF file + * and it contains embedded build ID. + * + * Kernel will set this field to zero, if VMA has no backing file, + * backing file is not an ELF file, or ELF file has no build ID + * embedded. + * + * Build ID is a binary value (not a string). Kernel will set + * build_id_size field to exact number of bytes used for build ID. + * If build ID is requested and present, but needs more bytes than + * user-supplied maximum buffer size (see build_id_addr field below), + * -E2BIG error will be returned. + * + * If this field is set to non-zero value, build_id_addr should point + * to valid user space memory buffer of at least build_id_size bytes. + * If set to zero, build_id_addr should be set to zero as well + */ + __u32 build_id_size; /* in/out */ /* * User-supplied address of a buffer of at least vma_name_size bytes * for kernel to fill with matched VMA's name (see vma_name_size field @@ -519,6 +539,14 @@ struct procmap_query { * Should be set to zero if VMA name should not be returned. */ __u64 vma_name_addr; /* in */ + /* + * User-supplied address of a buffer of at least build_id_size bytes + * for kernel to fill with matched VMA's ELF build ID, if available + * (see build_id_size field description above for details). + * + * Should be set to zero if build ID should not be returned. + */ + __u64 build_id_addr; /* in */ }; #endif /* _UAPI_LINUX_FS_H */
The need to get ELF build ID reliably is an important aspect when dealing with profiling and stack trace symbolization, and /proc/<pid>/maps textual representation doesn't help with this. To get backing file's ELF build ID, application has to first resolve VMA, then use it's start/end address range to follow a special /proc/<pid>/map_files/<start>-<end> symlink to open the ELF file (this is necessary because backing file might have been removed from the disk or was already replaced with another binary in the same file path. Such approach, beyond just adding complexity of having to do a bunch of extra work, has extra security implications. Because application opens underlying ELF file and needs read access to its entire contents (as far as kernel is concerned), kernel puts additional capable() checks on following /proc/<pid>/map_files/<start>-<end> symlink. And that makes sense in general. But in the case of build ID, profiler/symbolizer doesn't need the contents of ELF file, per se. It's only build ID that is of interest, and ELF build ID itself doesn't provide any sensitive information. So this patch adds a way to request backing file's ELF build ID along the rest of VMA information in the same API. User has control over whether this piece of information is requested or not by either setting build_id_size field to zero or non-zero maximum buffer size they provided through build_id_addr field (which encodes user pointer as __u64 field). This is a completely optional piece of information, and so has no performance implications for user cases that don't care about build ID, while improving performance and simplifying the setup for those application that do need it. Kernel already implements build ID fetching, which is used from BPF subsystem. We are reusing this code here, but plan a follow up changes to make it work better under more relaxed assumption (compared to what existing code assumes) of being called from user process context, in which page faults are allowed. BPF-specific implementation currently bails out if necessary part of ELF file is not paged in, all due to extra BPF-specific restrictions (like the need to fetch build ID in restrictive contexts such as NMI handler). Signed-off-by: Andrii Nakryiko <andrii@kernel.org> --- fs/proc/task_mmu.c | 25 ++++++++++++++++++++++++- include/uapi/linux/fs.h | 28 ++++++++++++++++++++++++++++ 2 files changed, 52 insertions(+), 1 deletion(-)